drm/i915: Don't check tc_mode unless dealing with a TC PHY

drm/i915: Don't check tc_mode unless dealing with a TC PHY

We shouldn't really trust tc_mode on non-TC PHYs since we never
initialize it explicitly. So let's check for the PHY type first.
Fortunately TC_PORT_TBT_ALT happens to be zero so I don't think
there's an actual bug here, just a possibility for a future one
if someone rearranges the enum values.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210128155948.13678-5-ville.syrjala@linux.intel.com
Reviewed-by: Imre Deak <imre.deak@intel.com>