drm/fb-helper: reject any changes to the fbdev

drm/fb-helper: reject any changes to the fbdev

The current fbdev emulation does not allow to push back changes in
width, height or depth to KMS, hence reject any changes with an
error. This makes sure that fbdev ioctl's fail properly and user
space does not assume that changes succeeded.

Change-Id: Iaefb728b04d4599ef5bd6f32b37dd9dcab830989
Signed-off-by: Stefan Agner <stefan@agner.ch>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20161011231504.17688-1-stefan@agner.ch