drm/i195: Make the async flip VT-d workaround dynamic

drm/i195: Make the async flip VT-d workaround dynamic

Since the VT-d vs. async flip issues are plaguing a wider range
of supported hw let's try to minimize the impact on normal
operation by flipping the relevant chicken bits on and off
as needed. I presume there is some power/perf impact on since
this is reducing some prefetching I think.

Cc: Karthik B S <[email protected]>
Signed-off-by: Ville Syrjälä <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]
Reviewed-by: Matt Roper <[email protected]>