drm/i915/dg2: Classify DG2 PHY types

Although the bspec labels four of DG2's outputs as "combo PHY," the
underlying PHYs in both cases are actually Synopsys PHYs that are
programmed completely differently than the traditional Intel "combo" PHY
units.  As such, we don't want intel_phy_is_combo to take us down legacy
programming paths, so just return false from it on DG2.  Instead add a
new intel_phy_is_snps() that will return true for all DG2 PHYs.

Cc: Anusha Srivatsa <anusha.srivatsa@intel.com>
Cc: Matt Atwood <matthew.s.atwood@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Matt Atwood <matthew.s.atwood@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210714031540.3539704-46-matthew.d.roper@intel.com