drm/radeon: drop radeon_fb_helper_set_par

It was just a wrapper around drm_fb_helper_set_par that
called cursor_set2 in addition.  Now that the core handles
this, drop this radeon specific version.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>