drm/i915/tc: Factor out helpers converting HPD mask to TC mode

Factor out helpers used later in the patchset to convert an HPD
status mask to TC mode or target TC mode.

No functional changes.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230316131724.359612-7-imre.deak@intel.com