drm/i915: enable trickle feed on Haswell

We shouldn't disable the trickle feed bits on Haswell. Our
documentation explicitly says the trickle feed bits of PRI_CTL and
CUR_CTL should not be programmed to 1, and the hardware engineer also
asked us to not program the SPR_CTL field to 1. Leaving the bits as 1
could cause underflows.

Reported-by: Arthur Runyan <arthur.j.runyan@intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>