drm/nouveau/kms/nv04-nv4x: fix exposed format list

drm_crtc_init exposes the XRGB8888 and ARGB8888 formats. In actuality,
ARGB8888's 32-bit depth messes up some formulas that weren't meant for
it, and the alpha is fairly meaningless for the primary plane.

The modesetting logic appears to be fully prepared for RGB565 as well as
XRGB1555 however, as tested with modetest.

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