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: NAkash Goel <akash.goel@intel.com>
Signed-off-by: NSourab Gupta <sourab.gupta@intel.com>
Reviewed-by: NVille Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: NChris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>