drm/i915: Nuke MI_ARB_STATE save/restore

drm/i915: Nuke MI_ARB_STATE save/restore

Originally added in commit e86e03bac166 ("drm/i915 more registers for
S3 (DSPCLK_GATE_D, CACHE_MODE_0, MI_ARB_STATE)") to fix some underruns.
I suspect that was due to the trickle feed settings getting clobbered
during suspend. We've been disabling trickle feed explicitly since
commit c1f93b703f2b ("drm/i915: Disable trickle feed via MI_ARB_STATE
for the gen4") so this magic save/restore should no longer be needed.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200908140210.31048-3-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>