drm: Make drm_crtc_convert_{umode, to_umode} static and constify their params

drm_crtc_convert_umode() and drm_crtc_convert_to_umode() are never
used outside drm_crtc.c, so make them static. Also make the input
mode structure const for both functions.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>