drm/i915/vlv:Implement the WA 'WaDisable_RenderCache_OperationalFlush'

On Gen4+ platforms (except BDW), Render Cache Operational flush
cannot be enabled.
This WA is apparently required for all Gen4+ platforms,except BDW.
In BDW, the bit has been repurposed otherwise.
This has been tested only on vlv.

v2: Corrected the code regarding the wrong usage of
MASKED_BIT_DISABLE (Chris)

v3: Enhancing the scope of WA to Gen4+ platforms except BDW (Ville)

v4: Adding WA for g4x, crestline, broadwater (Ville)

Signed-off-by: Akash Goel <akash.goel@intel.com>
Signed-off-by: Sourab Gupta <sourab.gupta@intel.com>
Reviewed-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>