i965/skl: Don't use the PMA depth stall workaround

i965/skl: Don't use the PMA depth stall workaround

The PMA depth stall must be enabled (optimization turned off) under certain
circumstances on gen8. This was supposedly fixed for Gen9, which means we do not
need to check, or toggle the state. The hardware is supposed to enable the
hardware optimization by default, unlike BDW, so we also don't need to set it at
init. For whatever reason this improves stability on ETQW with the bug mentioned
below.

References: https://bugs.freedesktop.org/show_bug.cgi?id=89039 (doesn't fix)
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Tested-by: Anuj Phogat <anuj.phogat@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>