drm/mipi-dbi: Move drm_dev_{enter, exit}() out from fb_dirty functions

Call drm_dev_enter() and drm_dev_exit() in the outer-most callbacks
of the modesetting pipeline. If drm_dev_enter() fails, the driver can
thus avoid unnecessary work.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Noralf Trønnes <noralf@tronnes.org>
Tested-by: Javier Martinez Canillas <javierm@redhat.com>
Tested-by: Noralf Trønnes <noralf@tronnes.org> # drm/tiny/mi0283qt
Link: https://patchwork.freedesktop.org/patch/msgid/20221202125644.7917-9-tzimmermann@suse.de