drm/amdgpu: user BO priority instead of self coding it (v2)

Keeping groups of BOs on the LRU is to time consuming on command submission.

Instead use the newly added BO priority to give a certain eviction order.

v2: agd: trivial warning fix

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Roger.He <Hongbo.He@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>