drm/amdgpu: double the priority of kernel allocations

Give kernel allocations a higher priority cause it is often
more work to swap them back in.

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>