drm/i915/panel: register drm_panel and call prepare/unprepare for ICL+ DSI

drm/i915/panel: register drm_panel and call prepare/unprepare for ICL+ DSI

Allocate and register a drm_panel so that drm_panel_followers can find
the panel. Pass the drm_connector::kdev device to drm_panel allocation
for matching. That's only available after drm_sysfs_connector_add(), so
we need to postpone the drm_panel allocation until .late_register()
hook.

The drm_panel framework is moving towards devm_drm_panel_alloc(). It
requires a wrapper struct, and struct intel_panel would be the natural
candidate. However, we can't postpone its allocation until
.late_register(), so we have to use __devm_drm_panel_alloc() directly
for now.

Call the drm_panel_prepare() and drm_panel_unprepare() functions for
ICL+ DSI, so that followers get notified of the panel power state
changes. This can later be expanded to VLV+ DSI and eDP.

Cc: Maxime Ripard <mripard@kernel.org>
Cc: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Cc: Lee Shawn C <shawn.c.lee@intel.com>
Tested-by: Lee Shawn C <shawn.c.lee@intel.com>
Reviewed-by: Maxime Ripard <mripard@kernel.org>
Link: https://lore.kernel.org/r/13d15c1414e65ffb21944d66e2820befdab54e98.1749199013.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>