drm/ast: Style cleanups in plane code

Rename some of the variables in the plane code to better reflect the
old and new state during checks and updates. Change some indention as
well. No functional changes.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Jocelyn Falempe <jfalempe@redhat.com>
Tested-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221013112923.769-7-tzimmermann@suse.de