drm: simple_kms_helper: Add prepare_fb and cleanup_fb hooks

Add .prepare_fb and .cleanup_fb plane hooks into the drm_simple_kms.
These can be used by drivers to call ie. the drm_fb_cma_setup_fence()
helper.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Noralf Trønnes <noralf@tronnes.org>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: David Airlie <airlied@linux.ie>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20161002170124.6099-1-marex@denx.de