drm/amdgpu: remove set but not used variable 'priority'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/gpu/drm/amd/amdgpu/amdgpu_cs.c:1211:26: warning: variable ‘priority’ set but not used

It is not used since commit 33abcb1f ("drm/amdgpu:
set compute queue priority at mqd_init")

Reviewed-by: Christian König <christian.koenig@amd.com>
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Zheng Bin <zhengbin13@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>