Because, there is no reason for it not to be const.

v1: original
v2: fix compile break in vmwgfx, and couple related cleanups suggested
    by Ville Syrjälä
Signed-off-by: NRob Clark <robdclark@gmail.com>
Reviewed-by: NVille Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: NAlex Deucher <alexander.deucher@amd.com>
Signed-off-by: NDave Airlie <airlied@redhat.com>