Add {begin,end}_fb_access helpers to run at the beginning and end of
an atomic commit. The begin_fb_access helper acquires resources that
are necessary to perform the atomic commit. It it similar to prepare_fb,
except that the resources are to be released at the end of the commit.
Resources acquired by prepare_fb are held until after the next pageflip.

The end_fb_access helper performs the corresponding resource cleanup.
Atomic helpers call it with the new plane state. This is different from
cleanup_fb, which releases resources of the old plane state.

v2:
	* fix typos in commit message (Javier)
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221025101737.8874-2-tzimmermann@suse.de