drm: Nuke ums vgaarb support

i915, nouveau (ever since merged to upstream) and radeon all lack ums
support in upstream. No point keeping the ums vgaarb support around.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170125062657.19270-12-daniel.vetter@ffwll.ch