drm/amdgpu: Use gmc_vram_full_visible in vram_mgr_bo_invisible_size

Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>