drm/i915/display: ignore long HPDs based on a flag

Some panels generate long HPD events even while connected to
the port. This cause some unexpected CI execution issues. A
new flag is added to track if such spurious long HPDs can be
ignored and are not processed further if the flag is set.
Debugfs entry is added to control the ignore long hpd flag.

v2: Address patch styling comments (Jani Nikula)

v3: Ignoring the HPD moved to hotplug handler and now applies
    to all types of outputs (Imre Deak)

v4: use debugfs_create_bool and squash patches (Jani Nikula)

Signed-off-by: Vinod Govindapillai <vinod.govindapillai@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230215083832.287519-2-vinod.govindapillai@intel.com