drm/i915: Factor out a helper for handling atomic modeset locks/state

This patch simplifying the handling of modeset locks and atomic state
for an atomic commit is based on

https://lore.kernel.org/all/20210715184954.7794-2-ville.syrjala@linux.intel.com/

adding the helper to i915. I find this approach preferrable than
open-coding the corresponding steps (fixed for me an atomic
state reset during a DEADLK retry, which I missed in the open-coded
version) and also better than the existing
DRM_MODESET_LOCK_ALL_BEGIN/END macros for the reasons described in the
above original patchset.

This change takes the helper into use only for atomic commits during DDI
hotplug handling, as a preparation for a follow-up patch adding a
similar commit started from the same spot. Other places doing a
driver-internal atomic commit is to be converted by a follow-up
patchset.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230510103131.1618266-13-imre.deak@intel.com