drm/amdgpu: fix start calculation in amdgpu_vram_mgr_new

We still need to calculate a virtual start address for the resource to
aid checking of it is visible or not. Only matters on small BAR systems,
but better save than sorry.

Signed-off-by: Christian König <christian.koenig@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Arunpravin Paneer Selvam <Arunpravin.PaneerSelvam@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220510113649.879821-1-christian.koenig@amd.com