drm/amd/display: minor cleanup of vm_setup

Use fb_start/end for consistency with gmc code for non-
XGMI systems, they are equivalent to vram_start/end.

Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>