drm/amdgpu: fix visible VRAM handling during faults

When we removed the hacky start code check we actually didn't took into
account that *all* VRAM pages needs to be CPU accessible.

Clean up the code and unify the handling into a single helper which
checks if the whole resource is CPU accessible.

The only place where a partial check would make sense is during
eviction, but that is neglitible.

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

 ("drm/amdgpu: Remove TTM resource->start visible VRAM condition v2")
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
CC: stable@vger.kernel.org