drm/i915: Introduce <platform>_hotplug_mask()

Pair each <platform>_hotplug_enables() function with
a corresponding <platform>_hotplug_mask() function so that
we can determine right bits to clear on a per hpd_pin basis.
We'll need this for turning on HPD sense for a specific
encoder rather than just all of them.

v2: Drop the unused 'i915' param (Jani)
v3: Drop the _foo_hotplug_enables() redirection too
v4: Deal with mtp

Cc: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Vinod Govindapillai <vinod.govindapillai@intel.com> #v3
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230417131728.7705-2-ville.syrjala@linux.intel.com