drm/i915/display: Cleanup intel_phy_is_combo()

Cleanup the intel_phy_is_combo
to accommodate for cases where combo phy is not available.

v2: retain comment that explains DG2 returning false from
intel_phy_is_combo() (Arun)

Cc: Arun R Murthy <arun.r.murthy@intel.com>
Cc: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Anusha Srivatsa <anusha.srivatsa@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Arun R Murthy <arun.r.murthy@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220725225028.98612-1-anusha.srivatsa@intel.com