drm/i915: Re-init clock gating on coming out of PC8+

drm/i915: Re-init clock gating on coming out of PC8+

PC8+ clobbers a bunch of displays registers which need to
be restored by hand or else we lost a bunch of workarounds.
The important ones for us are at least CHICKEN_PAR2* and
CHICKEN_PIPESL*.

Curiously at least some CHICKEN_PAR1* registers
are preserved by the hardware/firmware. Unfortunately Bspec
doens't really specify what gets clobbered vs. preserved
so further reverse engieering might be warranted to figure
out the specifics.

Note that PCH_LP_PARTITION_LEVEL_DISABLE is also set by
lpt_init_clock_gating() so the rmw in hsw_disable_pc8()
is now redundant. Remove it.

TODO: I suspect most gt stuff doesn't need this and we should
      finish moving all of them from init_clock_gating() to
      a more appropriate place...

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230609141404.12729-2-ville.syrjala@linux.intel.com
Reviewed-by: Jouni Högander <jouni.hogander@intel.com>