drm/imx: make fbdev support really optional

Currently enabling Freescale i.MX DRM support automatically pulls
in fbdev dependency. However this dep is unnecessary since
DRM core already handles this for us (DRM_FBDEV_EMULATION).
Signed-off-by: Tobias Jakobi <tjakobi@math.uni-bielefeld.de>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1468586897-32298-8-git-send-email-tjakobi@math.uni-bielefeld.de

drivers/gpu/drm/imx/Kconfig

 config DRM_IMX
 	tristate "DRM Support for Freescale i.MX"
 	select DRM_KMS_HELPER
-	select DRM_KMS_FB_HELPER
 	select VIDEOMODE_HELPERS
 	select DRM_GEM_CMA_HELPER
 	select DRM_KMS_CMA_HELPER