gpu: ipu-v3: add DRM dependency

The new PRE/PRG driver code causes a link failure when IPUv3 is built-in,
but DRM is built as a module:

drivers/gpu/ipu-v3/ipu-pre.o: In function `ipu_pre_configure':
ipu-pre.c:(.text.ipu_pre_configure+0x18): undefined reference to `drm_format_info'
drivers/gpu/ipu-v3/ipu-prg.o: In function `ipu_prg_format_supported':
ipu-prg.c:(.text.ipu_prg_format_supported+0x8): undefined reference to `drm_format_info'

Adding a Kconfig dependency on DRM means we don't run into this problem
any more. If DRM is disabled altogether, the IPUv3 driver is built
without PRE/PRG support.

Fixes: ea9c2605 ("gpu: ipu-v3: add driver for Prefetch Resolve Gasket")
Link: https://patchwork.kernel.org/patch/9636665/Signed-off-by: Arnd Bergmann <arnd@arndb.de>
[p.zabel@pengutronix.de: changed the dependency from DRM to DRM || !DRM,
 since the link failure only happens when DRM=m and IPUV3_CORE=y.
 Modified the commit message to reflect this.]
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>

drivers/gpu/ipu-v3/Kconfig

 config IMX_IPUV3_CORE
 	tristate "IPUv3 core support"
 	depends on SOC_IMX5 || SOC_IMX6Q || ARCH_MULTIPLATFORM
+	depends on DRM || !DRM # if DRM=m, this can't be 'y'
 	select GENERIC_IRQ_CHIP
 	help
 	  Choose this if you have a i.MX5/6 system and want to use the Image