drm/i915: Prevent HW access during init from connector get_modes hooks

drm/i915: Prevent HW access during init from connector get_modes hooks

Prevent accessing the HW from the get_modes hooks of connectors deriving
the mode list from the display's EDID. drm_edid_connector_add_modes()
will return the mode list based on the EDID which was cached during a
previous detection/get_modes call.

This also fixes the NULL deref problem (10085) which was
introduced/revealed by

commit bab87ef4db9a ("drm/i915: Disable hotplug detection handlers during driver init/shutdown")

After the above change MST connectors will not change state during
driver init/shutdown; thus some of these connectors with no I2C/DDC
adapter registered for them (since the given MST port has no sink
connected) may stay then in the 'unknown' connector status. The
get_modes() hook should not try to use the I2C/DDC adapter in this state
(which would lead to the above NULL deref) which this patch ensures.

v2:
- Remove the redundant check from intel_crt_ddc_get_modes().
- Rebase on latest drm-tip.
- Add Fixes: line / related commit notes.

Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/10085
Fixes: bab87ef4db9a ("drm/i915: Disable hotplug detection handlers during driver init/shutdown")
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Jouni Högander <jouni.hogander@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240212175237.2625812-2-imre.deak@intel.com