drm/i915/mtl: Fix Wa_16015201720 implementation

The commit 2357f2b2 ("drm/i915/mtl: Initial display workarounds")
extended the workaround Wa_16015201720 to MTL. However the registers
that the original WA implemented moved for MTL.

Implement the workaround with the correct register.

v3: Skip clock gating for pipe C, D DMC's and fix the title

Fixes: 2357f2b2

 ("drm/i915/mtl: Initial display workarounds")
Cc: Matt Atwood <matthew.s.atwood@intel.com>
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Signed-off-by: Radhakrishna Sripada <radhakrishna.sripada@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230301201053.928709-2-radhakrishna.sripada@intel.com