drm/atomic: Unconditionally call prepare_fb.

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 fcc60b413d14dd06ddbd79ec50e83c4fb2a097ba
Author: Keith Packard <[email protected]>
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 <[email protected]>
Acked-by: Laurent Pinchart <[email protected]>
Signed-off-by: Daniel Vetter <[email protected]>
Link: http://patchwork.freedesktop.org/patch/msgid/[email protected]