drm: Extract drm_plane.[hc]

Just pure code movement, cleanup and polish will happen in later
patches.

v2: Don't forget all the ioctl! To extract those cleanly I decided to
put check_src_coords into drm_framebuffer.c (and give it a
drm_framebuffer_ prefix), since that just checks framebuffer
constraints.

v3: rebase over PAGE_FLIP_TARGET.

Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>

[seanpaul]
This patch as posted on the list was rebased on:

commit 6f00975c
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Sat Aug 20 12:22:11 2016 +0200

    drm: Reject page_flip for !DRIVER_MODESET

so as a result of moving the page_flip ioctl, this fix has
been rolled into this patch.

Signed-off-by: Sean Paul <seanpaul@chromium.org>