drm: Restore lost drm_framebuffer_unreference in drm_mode_page_flip_ioctl

drm: Restore lost drm_framebuffer_unreference in drm_mode_page_flip_ioctl

Commit 43968d7b806d ("drm: Extract drm_plane.[hc]") was not the simple
cut'n'paste we presumed, somehow it introduced a leak of the page flip
target's framebuffer.

Fixes: 43968d7b806d ("drm: Extract drm_plane.[hc]")
Signed-off-by: Chris Wilson <[email protected]>
Cc: Daniel Vetter <[email protected]>
Cc: Sean Paul <[email protected]>
Signed-off-by: Daniel Vetter <[email protected]>
Link: http://patchwork.freedesktop.org/patch/msgid/[email protected]
Signed-off-by: Dave Airlie <[email protected]>