drm/i915/xelpd: Fix unclaimed accesses while loading PIPEDMC-C/D

At the moment on DG2 at least loading the DMC firmware's PIPEDMC C and D
programs leads to sporadic unclaimed register accesses while programming
the initial state as described by the firmware's "MMIO init" table. This
will also lead to later unclaimed accesses for unrelated transcoder/pipe
registers backed by the pipe C and D display power wells.

Disabling the PIPEDMC clock gating during initialization - similarly to
Wa_16015201720 fixed this problem in my tests. While pipe A an B
requires the clock gating to be disabled all the time pipe C and D
requires this only while accessing the PIPEDMC registers.

Bspec: 49193
References: https://gitlab.freedesktop.org/drm/intel/-/issues/6526
References: https://gitlab.freedesktop.org/drm/intel/-/issues/6308


Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Arun R Murthy <arun.r.murthy@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220808103054.3586074-1-imre.deak@intel.com