drm/i915: Remove zombie async flip vt-d w/a

drm/i915: Remove zombie async flip vt-d w/a

This async flip vt-d w/a was moved to a different place in
commit 7d396cacaea6 ("drm/i195: Make the async flip VT-d workaround
dynamic") but the drm-intel-fixes cherry-pick commit b2d73debfdc1
("drm/i915: Extend the async flip VT-d w/a to skl/bxt") resurrected
the original code as well. So now we have this w/a in two places.
Remove the resurrected zombie code.

Not done as a revert to hopefully prevent any kind of
automagic stable backport.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211208150050.17230-1-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>