drm: fsl-dcu: select CONFIG_DRM_GEM_CMA_HELPER

drm: fsl-dcu: select CONFIG_DRM_GEM_CMA_HELPER

The fsl-dcu driver uses the gem_cma_helper code, but does
noto enforce enabling this through Kconfig:

arm-linux-gnueabi-ld: drivers/gpu/drm/fsl-dcu/fsl_dcu_drm_drv.o:(.rodata+0x32c): undefined reference to `drm_gem_cma_prime_import_sg_table'
arm-linux-gnueabi-ld: drivers/gpu/drm/fsl-dcu/fsl_dcu_drm_drv.o:(.rodata+0x334): undefined reference to `drm_gem_cma_dumb_create'
arm-linux-gnueabi-ld: drivers/gpu/drm/fsl-dcu/fsl_dcu_drm_plane.o: in function `fsl_dcu_drm_plane_atomic_update':
fsl_dcu_drm_plane.c:(.text+0x144): undefined reference to `drm_fb_cma_get_gem_obj'

Add the same 'select' that is used in other such drivers.

Fixes: 09717af7d13d ("drm: Remove CONFIG_DRM_KMS_CMA_HELPER option")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20211204174637.1160725-1-arnd@kernel.org