drm/amdgpu: add helper function for gfx queue/bitmap transition

Similar to what we do for compute already.

Signed-off-by: Hawking Zhang <Hawking.Zhang@amd.com>
Reviewed-by: Jack Xiao <jack.xiao@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Acked-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>