compositor-drm: Refactor destroy drm_fb function

compositor-drm: Refactor destroy drm_fb function

The drm_fb destroy callback to mostly the same thing regardless of
whether the buffer is a dumb buffer or gbm buffer. This patch refactors
the common parts into a new function that can be called for both cases.

[daniels: Rebased on top of fb->fd changes, cosmetic changes.]

Signed-off-by: Tomohito Esaki <etom@igel.co.jp>
Signed-off-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>