drm/amdgpu: Use ttm_pages_limit to override vram reporting

On GFXIP9.4.3 APU, allow the memory reporting as per the ttm pages
limit in NPS1 mode.

Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Rajneesh Bhardwaj <rajneesh.bhardwaj@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>