drm/amdgpu: use adev_to_drm for consistency when accessing drm_device

adev_to_drm is used everywhere, so improve recent changes
when accessing drm_device pointer from amdgpu_device.

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