drm/crtc: Remove duplicated ioctl code

Make drm_mode_add_fb() call drm_mode_add_fb2() after converting its
args to the new internal format, instead of duplicating code.

Also picks up a lot more error checking, which the legacy modes
should pass after being converted to the new format.

Signed-off-by: Chuck Ebbert <cebbert.lkml@gmail.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>