drm/i915/ehl: Check PHY type before reading DPLL frequency

intel_ddi_clock_get() tests the DPLL ID against DPLL_ID_ICL_TBTPLL (2)
to determine whether to try to descend into a TBT-specific handler.
However this test will also be true when DPLL4 on EHL is used since that
shares the same DPLL ID (2).

Add an extra check to ensure the PHY is actually a Type-C PHY before
descending into the TBT handling.  This should ensure EHL still takes
the correct code path and somewhat future-proof the code as well.

v2: Drop the gen+ check since only gen11+ platforms can have Type-C
    outputs.  (Imre)

Cc: José Roberto de Souza <jose.souza@intel.com>
Cc: Imre Deak <imre.deak@intel.com>
Closes: https://gitlab.freedesktop.org/drm/intel/issues/1369


Fixes: 45e4728b ("drm/i915: Move DPLL frequency calculation to intel_dpll_mgr.c")
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200303195043.959913-1-matthew.d.roper@intel.com