drm/amdgpu: remove invalid resource->start check v2

The majority of those where removed in the commit aed01a68


("drm/amdgpu: Remove TTM resource->start visible VRAM condition v2")

But this one was missed because it's working on the resource and not the
BO. Since we also no longer use a fake start address for visible BOs
this will now trigger invalid mapping errors.

v2: also remove the unused variable

Signed-off-by: Christian König <christian.koenig@amd.com>
Fixes: aed01a68

 ("drm/amdgpu: Remove TTM resource->start visible VRAM condition v2")
CC: stable@vger.kernel.org
Acked-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>