When we first create a drm_fb from a weston_buffer, cache it and keep it
alive as long as the buffer lives. This allows us to reuse the gbm_bo
and kernel-side DRM framebuffer, rather than constantly creating and
destroying them at every repaint. The overhead of doing so (e.g. MMU
updates) can be significant on some platforms.

Signed-off-by: Daniel Stone <daniels@collabora.com>