drm/i915: Remove the enabling of VS_TIMER_DISPATCH bit in MI MODE reg

This patch Removes the VS_TIMER_DISPATCH bit enable in MI MODE reg for
platforms > Gen6.
VS_TIMER_DISPATCH bit enable was earlier required as a part of
WA 'WaTimedSingleVertexDispatch', which is now applicable only to
platforms < Gen7.

v2: Enhancing the scope of the patch to full Gen7 (Chris)

v3: Modifying the WA condition to the cover the applicable platforms,
and adding the WA name in comments. (Ville)

Signed-off-by: Akash Goel <akash.goel@intel.com>
Signed-off-by: Sourab Gupta <sourab.gupta@intel.com>
Tested-by: Chris Wilson <chris@chris-wilson.co.uk> # ivb, hsw -Chris
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>