drm/nouveau: Replace drm_framebuffer_{un/reference} with put, get functions

This patch unifies the naming of DRM functions for reference counting
of struct drm_framebuffer. The resulting code is more aligned with the
rest of the Linux kernel interfaces.

Signed-off-by: Thomas Zimmermann <tdz@users.sourceforge.net>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>