drm/amdgpu: use adev_to_drm to get drm_device pointer

Updated for consistency when accessing drm_device from amdgpu driver.

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