drm/radeon/kms: add gui_idle callback

Check to see if the GUI engine and related blocks
(2D, 3D, CP, etc) are idle or not.  There are a number
of cases when we need to know if the drawing engine
is busy.

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>