drm: Simplify core vs. drv ioctl handling

Now that cmd_drv is gone the handling for core and driver ioctls is
mostly identical, so eliminate the duplication. Also take the
opportunity to simplify the range checks to be less cluttered.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>