drm/nouveau/kms/nv50-: fix panel scaling

Under certain circumstances, encoder atomic_check() can be entered
without adjusted_mode having been reset to the same as mode, which
confuses the scaling logic and can lead to a misprogrammed display.

Fix this by checking against the user-provided mode directly.

Link: https://bugs.freedesktop.org/show_bug.cgi?id=108615
Link: https://gitlab.freedesktop.org/xorg/driver/xf86-video-nouveau/issues/464


Signed-off-by: Ben Skeggs <bskeggs@redhat.com>