drm/i915/bdw: Cleanup pre prod workarounds

as these have been fixed in production hw and hurt performance
if applied.

v2: adjust requested ring space (Ville)

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=83482


Tested-by: zhoujian <jianx.zhou@intel.com>
Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>