drm/amdgpu: use drm_device pointer directly rather than convert again

The convert from adev is redundant.

Signed-off-by: Guchun Chen <guchun.chen@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>