drm/i915/ehl: Don't program PHY_MISC on EHL PHY C

drm/i915/ehl: Don't program PHY_MISC on EHL PHY C

Although EHL added a third combo PHY, no PHY_MISC register was added for
PHY C.  The bspec indicates that there's no need to program the "DE to
IO Comp Pwr Down" setting for this PHY that we usually need to set in
PHY_MISC.

v2:
 - Add IS_ELKHARTLAKE() guards since future platforms that have a PHY C
   are likely to reinstate the PHY_MISC register.  (Jose)
 - Use goto's to skip PHY_MISC programming & minimize code deltas. (Jose)

Bspec: 33148
Cc: José Roberto de Souza <jose.souza@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190626000352.31926-4-matthew.d.roper@intel.com
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>