drm/amdgpu: Remove superfluous NULL check

The DRM device is a static member of
the amdgpu device structure and as such
always exists, so long as the PCI and
thus the amdgpu device exist.

Signed-off-by: Luben Tuikov <luben.tuikov@amd.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>