drm/amdgpu: Consolidate visible vs. real vram check v2.

Move all instnaces of this check into a function in amdgpu_gmc.h
Rename the original function to a more proper name.

v2:
Add more places to cleanup.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>