drm/radeon/kms: fix GTT/VRAM overlapping test

GTT/VRAM overlapping test had a typo which leaded to not
detecting case when vram_end > gtt_end. This patch fix the
logic and should fix #16574

Signed-off-by: Jerome Glisse <jglisse@redhat.com>
Cc: stable@kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>