drm/i915: Enabling the TLB invalidate bit in GFX Mode register

This patch Enables the bit for TLB invalidate in GFX Mode register
for Gen7.

According to bspec,  When enabled this bit limits the invalidation
of the TLB only to batch buffer boundaries, to pipe_control
commands which have the TLB invalidation bit set and sync flushes.
If disabled, the TLB caches are flushed for every full flush of
the pipeline.

Tested only on vlv platform. Chris has tested on ivb and hsw
platforms.

v2: Adding the explicit enabling of this bit for all Gen7 platforms
instead of only vlv (Chris)

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>
[danvet: Add w/a markers as suggested by Ville.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>