drm/i915/dg1: Don't program PHY_MISC for PHY-C and PHY-D

The only bit we use in PHY_MISC is DE_IO_COMP_PWR_DOWN, and the bspec
details for that bit tell us that it need only be set for PHY-A and
PHY-B.  It also turns out that there isn't even an instance of the
PHY_MISC register for PHY-D on this platform.  Let's extend the EHL/RKL
logic that conditionally skips PHY_MISC usage to DG1 as well.

Bspec: 50107
Cc: Aditya Swarup <aditya.swarup@intel.com>
Cc: Clinton Taylor <Clinton.A.Taylor@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Anusha Srivatsa <anusha.srivatsa@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201007002210.3678024-6-lucas.demarchi@intel.com