drm/mode: Validate modes inside drm_crtc_convert_umode

The only user of convert_umode was also performing mode validation, so
do that in the same place.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Tested-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>