drm/amdgpu: Get DRM dev from adev by inline-f

Add a static inline adev_to_drm() to obtain
the DRM device pointer from an amdgpu_device pointer.

Signed-off-by: Luben Tuikov <luben.tuikov@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>