drm/nouveau/disp/nv50-: fix center/aspect-corrected scaling

[ Upstream commit 533f4752407543f488a9118d817b8c504352b6fb ]

Previously center scaling would get scaling applied to it (when it was
only supposed to center the image), and aspect-corrected scaling did not
always correctly pick whether to reduce width or height for a particular
combination of inputs/outputs.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=110660


Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>