drm/amdgpu: switch to drm_*{get,put} helpers

drm_*_reference() and drm_*_unreference() functions are just
compatibility alias for drm_*_get() and drm_*_put() and should not be
used by new code. So convert all users of compatibility functions to use
the new APIs.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Cihangir Akturk <cakturk@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>