drm/atomic: Unconditionally call prepare_fb.

Atomic drivers may set properties like rotation on the same fb, which
may require a call to prepare_fb even when framebuffer stays identical.

Instead of handling all the special cases in the core, let the driver
decide when prepare_fb and cleanup_fb are noops.

This is a revert of:

commit fcc60b41
Author: Keith Packard <keithp@keithp.com>
Date:   Sat Jun 4 01:16:22 2016 -0700

    drm: Don't prepare or cleanup unchanging frame buffers [v3]

The original commit mentions that this prevents waiting in i915 on all
previous rendering during cursor updates, but there are better ways to
fix this.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/6d82f9b6-9d16-91d1-d176-4a37b09afc44@linux.intel.com