drm: Use stdint types for consistency

The rest of the code uses stdint types, so use them in
drm_property_change_is_valid() as well.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>