drm: Make some modes const when iterating through them

valid_inferred_mode() don't change the modes over which it iterates,
so make the iterator 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>