drm/cirrus: Move drm_dev_{enter, exit}() into DRM helpers

Call drm_dev_enter() and drm_dev_exit() immediately after entering
cirrus' DRM helper functions. Remove these calls from other functions.
Each enter/exit block in the DRM helpers covers the full hardware
update. No functional changes.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230215161517.5113-5-tzimmermann@suse.de