drm/tinydrm/Kconfig: drivers: Select BACKLIGHT_CLASS_DEVICE

The mipi_dbi helper is missing a dependency on DRM_KMS_HELPER and putting
that in revealed this problem:

drivers/video/fbdev/Kconfig:12:error: recursive dependency detected!
drivers/video/fbdev/Kconfig:12: symbol FB is selected by DRM_KMS_FB_HELPER
drivers/gpu/drm/Kconfig:75:     symbol DRM_KMS_FB_HELPER depends on DRM_KMS_HELPER
drivers/gpu/drm/Kconfig:69:     symbol DRM_KMS_HELPER is selected by TINYDRM_MIPI_DBI
drivers/gpu/drm/tinydrm/Kconfig:11:     symbol TINYDRM_MIPI_DBI is selected by TINYDRM_HX8357D
drivers/gpu/drm/tinydrm/Kconfig:15:     symbol TINYDRM_HX8357D depends on BACKLIGHT_CLASS_DEVICE
drivers/video/backlight/Kconfig:144:    symbol BACKLIGHT_CLASS_DEVICE is selected by FB_BACKLIGHT
drivers/video/fbdev/Kconfig:187:        symbol FB_BACKLIGHT depends on FB

A symbol that selects DRM_KMS_HELPER can not depend on
BACKLIGHT_CLASS_DEVICE. The reason for this is that DRM_KMS_FB_HELPER
selects FB instead of depending on it.

The tinydrm drivers have somehow gotten away with depending on
BACKLIGHT_CLASS_DEVICE because DRM_TINYDRM selects DRM_KMS_HELPER and the
drivers depend on that symbol.

An audit shows that all DRM drivers that select DRM_KMS_HELPER and use
BACKLIGHT_CLASS_DEVICE, selects it:
  DRM_TILCDC, DRM_GMA500, DRM_SHMOBILE, DRM_NOUVEAU, DRM_FSL_DCU,
  DRM_I915, DRM_RADEON, DRM_AMDGPU, DRM_PARADE_PS8622

Documentation/kbuild/kconfig-language.txt has a note regarding select:
1. 'select should be used with care since it doesn't visit dependencies.'
   This is not a problem since BACKLIGHT_CLASS_DEVICE doesn't have any
   dependencies.
2. 'In general use select only for non-visible symbols'
   BACKLIGHT_CLASS_DEVICE is user visible.

The real solution to this would be to have DRM_KMS_FB_HELPER depend on the
user visible symbol FB. That is a can of worms I'm not willing to tackle.
I fear that such a change will result in me handling difficult fallouts
for the next weeks. So I'm following DRM suite here.
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: David Lechner <david@lechnology.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190722104312.16184-7-noralf@tronnes.org

drivers/gpu/drm/tinydrm/Kconfig

@@ -14,8 +14,8 @@ config TINYDRM_MIPI_DBI
 config TINYDRM_HX8357D
 	tristate "DRM support for HX8357D display panels"
 	depends on DRM_TINYDRM && SPI
-	depends on BACKLIGHT_CLASS_DEVICE
 	select TINYDRM_MIPI_DBI
+	select BACKLIGHT_CLASS_DEVICE
 	help
 	  DRM driver for the following HX8357D panels:
 	  * YX350HV15-T 3.5" 340x350 TFT (Adafruit 3.5")
@@ -35,8 +35,8 @@ config TINYDRM_ILI9225
 config TINYDRM_ILI9341
 	tristate "DRM support for ILI9341 display panels"
 	depends on DRM_TINYDRM && SPI
-	depends on BACKLIGHT_CLASS_DEVICE
 	select TINYDRM_MIPI_DBI
+	select BACKLIGHT_CLASS_DEVICE
 	help
 	  DRM driver for the following Ilitek ILI9341 panels:
 	  * YX240QV29-T 2.4" 240x320 TFT (Adafruit 2.4")
@@ -46,8 +46,8 @@ config TINYDRM_ILI9341
 config TINYDRM_MI0283QT
 	tristate "DRM support for MI0283QT"
 	depends on DRM_TINYDRM && SPI
-	depends on BACKLIGHT_CLASS_DEVICE
 	select TINYDRM_MIPI_DBI
+	select BACKLIGHT_CLASS_DEVICE
 	help
 	  DRM driver for the Multi-Inno MI0283QT display panel
 	  If M is selected the module will be called mi0283qt.
@@ -78,8 +78,8 @@ config TINYDRM_ST7586
 config TINYDRM_ST7735R
 	tristate "DRM support for Sitronix ST7735R display panels"
 	depends on DRM_TINYDRM && SPI
-	depends on BACKLIGHT_CLASS_DEVICE
 	select TINYDRM_MIPI_DBI
+	select BACKLIGHT_CLASS_DEVICE
 	help
 	  DRM driver Sitronix ST7735R with one of the following LCDs:
 	  * JD-T18003-T01 1.8" 128x160 TFT