drm/amdgpu: use adev_to_drm to get drm device

adev_to_drm is used everywhere in amdgpu code, so modify
it to keep consistency.

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>