drm/i915: Nuke CACHE_MODE_0 save/restore

drm/i915: Nuke CACHE_MODE_0 save/restore

The CACHE_MODE_0 save/restore was added without explanation in
commit 1f84e550a870 ("drm/i915 more registers for S3 (DSPCLK_GATE_D,
CACHE_MODE_0, MI_ARB_STATE)"). If there are any bits we care about
those should be set explicitly during some appropriate init function.
Let's assume it's all good and just nuke this magic save/restore.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200908140210.31048-4-ville.syrjala@linux.intel.com
Acked-by: Jani Nikula <jani.nikula@intel.com>