drm: Constify the pretty-print functions

The structures and strings involved with various pretty-print functions
aren't meant to be modified, so make them all const. The exception is
drm_connector_enum_list which does get modified in drm_connector_init().

While at it move the drm_get_connector_status_name() prototype from
drmP.h to drm_crtc.h where it belongs.

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