drm/radeon/cik: add hw cursor support (v2)

CIK (DCE8) hw cursors are programmed the same as evergreen
(DCE4) with the following caveats:
- cursors are now 128x128 pixels
- new alpha blend enable bit

v2: rebase

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>