drm/i915/bxt: Add WaForceContextSaveRestoreNonCoherent

Note that we also need this for skl.

Signed-off-by: Nick Hoath <[e-mail]>
Reviewed-by: Imre Deak <[e-mail]>
[danvet: Note that we also need this for skl, requested by Imre.]
Signed-off-by: Daniel Vetter <[e-mail]>

drivers/gpu/drm/i915/intel_ringbuffer.c

···
 			GEN7_SBE_SS_CACHE_DISPATCH_PORT_SHARING_DISABLE);
 	}
 
+	/* WaForceContextSaveRestoreNonCoherent:bxt */
+	WA_SET_BIT_MASKED(HDC_CHICKEN0,
+			  HDC_FORCE_CONTEXT_SAVE_RESTORE_NON_COHERENT);
+
 	return 0;
 }