drm/panel: add panel driver for Xinpeng XPP055C272 panels

Base on the somewhat similar Rocktech driver but adapted for
panel-specific init of the XPP055C272.

changes in v5:
- drop error message when backlight not found, no other panel
  does that and if needed it should live in drm_panel_of_backlight
changes in v4:
  none
changes in v3:
- remove wrong negative sync flags from display-mode to fix a display
  artifact of the output getting move a tiny bit to the right
changes in v2:
- move to drm-panel-internal backlight handling (Sam)
- adapt to changes that happened to drm_panel structs+functions (Sam)
- sort includes (Sam)
- drop unnecessary DRV_NAME constant (Sam)
- do mipi_dsi_dcs_exit_sleep_mode and mipi_dsi_dcs_set_display_on
  in panel prepare (not init_sequence) to keep symmetric (Sam)
Signed-off-by: Heiko Stuebner <heiko.stuebner@theobroma-systems.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20191224112907.30758-3-heiko@sntech.de

drivers/gpu/drm/panel/Kconfig

@@ -366,4 +366,14 @@ config DRM_PANEL_TRULY_NT35597_WQXGA
 	help
 	  Say Y here if you want to enable support for Truly NT35597 WQXGA Dual DSI
 	  Video Mode panel
+
+config DRM_PANEL_XINPENG_XPP055C272
+	tristate "Xinpeng XPP055C272 panel driver"
+	depends on OF
+	depends on DRM_MIPI_DSI
+	depends on BACKLIGHT_CLASS_DEVICE
+	help
+	  Say Y here if you want to enable support for the Xinpeng
+	  XPP055C272 controller for 720x1280 LCD panels with MIPI/RGB/SPI
+	  system interfaces.
 endmenu

drivers/gpu/drm/panel/Makefile

@@ -39,3 +39,4 @@ obj-$(CONFIG_DRM_PANEL_TPO_TD028TTEC1) += panel-tpo-td028ttec1.o
 obj-$(CONFIG_DRM_PANEL_TPO_TD043MTEA1) += panel-tpo-td043mtea1.o
 obj-$(CONFIG_DRM_PANEL_TPO_TPG110) += panel-tpo-tpg110.o
 obj-$(CONFIG_DRM_PANEL_TRULY_NT35597_WQXGA) += panel-truly-nt35597.o
+obj-$(CONFIG_DRM_PANEL_XINPENG_XPP055C272) += panel-xinpeng-xpp055c272.o