drm/amdgpu: use adev_to_drm macro for consistency (v2)

Use adev_to_drm() to get to the drm_device pointer.

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