drm/i915: cleanup: drm_modeset_lock_all() --> DRM_MODESET_LOCK_ALL_BEGIN() part 2

As requested in Documentation/gpu/todo.rst, replace driver calls to
drm_modeset_lock_all() with DRM_MODESET_LOCK_ALL_BEGIN() and
DRM_MODESET_LOCK_ALL_END()

While the previous commit was a simple "search and replace", this time I
had to do a bit of refactoring as only one call to
DRM_MODESET_LOCK_ALL_BEGIN() is allowed inside one same function.

Signed-off-by: Fernando Ramos <greenfoo@u92.eu>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210924064324.229457-14-greenfoo@u92.eu