drm/i915: Drop unused _PORT3 and _PORT4 TC phy register offsets

All MG/DKL PHY register regions are evenly spaced offset-wise (0x168000,
0x169000, 0x16A000, 0x16B000) so the _MMIO_PORT() macro we use to access
their registers only needs the first two offsets.  We can drop the
_PORT3 and _PORT4 offsets which are never directly referenced.

Cc: Jani Nikula <jani.nikula@linux.intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220111051600.3429104-12-matthew.d.roper@intel.com