drm: omapdrm: Remove omap_crtc->full_update field

The full_update field is always set to true before calling
omap_crtc_appy(), resulting in its value always being true in the single
location where it is tested, in omap_crtc_pre_apply(). Remove it.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>