drm/amdgpu: reapply "fix start calculation in amdgpu_vram_mgr_new""

This re-applys commit 5e3f1e77.

The original problem this was reverted for was found and the correct fix
will be merged to drm-misc-next-fixes.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Arunpravin Paneer Selvam <Arunpravin.PaneerSelvam@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220714132315.587217-1-christian.koenig@amd.com