drm: Make drm_av_sync_delay() 'mode' argument const

drm_av_sync_delay() doesn't change the passed in mode, so make it const.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>