drm/nouveau: Use fb->format rather than drm_format_info()

Let's use the pointer to the format information cached under
drm_framebuffer rather than look it up manually.

Cc: Ben Skeggs <bskeggs@redhat.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1479498793-31021-28-git-send-email-ville.syrjala@linux.intel.com


Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>