compositor-drm: Rename current/next FB members

compositor-drm: Rename current/next FB members

'next' is used as a framebuffer which has either been rendered but not
had a configuration request (pageflip or CRTC set) applied to it, or
when for a framebuffer that has had configuration requested but not
applied (delayed pageflip where the event has not been applied).

'current' is used as the last framebuffer for which we know
configuration has been fully applied, i.e. CRTC set executed or pageflip
requested and event received.

Rename these members to fb_current and fb_pending, doing some small
reordering of drm_output whilst in the vicinity.

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