drm: Constify some function arguments

None of drm_mode_debug_printmodeline(), drm_mode_equal(), drm_mode_width()
or drm_mode_height() change the mode passed in, so make the arguments
const.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>