drm/i915: Don't set DPFC_HT_MODIFY bit on CTG/ILK/SNB

The ILK/SNB docs don't really mention the the DPFC_HT_MODIFY bit.
CTG docs clearly state that it should be set only when tracking
back buffer modification in persistent mode. The bit is supposed
to be set by software after the first CPU modification to the
back buffer, and it would get automagically cleared by the hardware
on the next page flip.

Since we only track front buffer modification we don't need to set
this bit. GTT modification tracking still appears to work on ILK
and SNB with the bit unset. I don't have a CTG to verify how that
behaves.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>