drm/nouveau: by default use low bpp framebuffer on low memory cards

Framebuffer's BPP is not that important but can waste significant part
of memory on low-VRAM cards. Lower it to 8bpp on < 32MB cards and to
16bpp on 64MB cards. It can still be overridden by video= option.

Signed-off-by: Marcin Slusarz <marcin.slusarz@gmail.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>