drm: Add "prefix" parameter to drm_rect_debug_print()

Allow the caller to specify a "prefix" string to drm_rect_debug_print()
to make it easier to see which drm_rect is being printed.

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