Add forward declaration and pull in include
file to make drm_framebuffer.h self contained.

While add it order include files alphabetically.

The use of TASK_COMM_LEN is the reason for including sched.h.
I could not see any good way to avoid this dependency,
and users of drm_framebuffer.comm already use
TASK_COMM_LEN to check for length etc.

v2:
- Added forward declaration of drm_gem_object (Noralf)
- Added ack from Noralf
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Noralf Trønnes <noralf@tronnes.org>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <maxime.ripard@bootlin.com>
Cc: Sean Paul <sean@poorly.run>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20190108192939.15255-4-sam@ravnborg.org