drm/i915: Provide a device level .mode_valid() hook

We never support certain mode flags etc. Reject those early on in the
mode_config.mode_valid() hook. That allows us to remove some duplicated
checks from the connector .mode_valid() hooks, and it guarantees that
we never see those flags even from user mode as the
mode_config.mode_valid() hooks gets executed for those as well.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171114183258.16976-11-ville.syrjala@linux.intel.com
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>