drm/i915/vlv: Modifying WA 'WaDisableL3Bank2xClockGate for vlv

For disabling L3 clock gating we need to set bit 25 of MMIO
register 940c. Earlier this was being done by just writing 1
into bit 25 and resetting all other bits.
This patch modifies the routine to read-modify-write of the
register, so that the values of other bits are not destroyed.

v2: Modifying the comments and the patch commit message (Chris)
Signed-off-by: Akash Goel <akash.goel@intel.com>
Signed-off-by: Sourab Gupta <sourab.gupta@intel.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
[danvet: Apply checkpatch fixup.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_pm.c

@@ -5348,8 +5348,11 @@ static void valleyview_init_clock_gating(struct drm_device *dev)
 	I915_WRITE(GEN6_UCGCTL2,
 		   GEN6_RCZUNIT_CLOCK_GATE_DISABLE);
 
-	/* WaDisableL3Bank2xClockGate:vlv */
-	I915_WRITE(GEN7_UCGCTL4, GEN7_L3BANK2X_CLOCK_GATE_DISABLE);
+	/* WaDisableL3Bank2xClockGate:vlv
+	 * Disabling L3 clock gating- MMIO 940c[25] = 1
+	 * Set bit 25, to disable L3_BANK_2x_CLK_GATING */
+	I915_WRITE(GEN7_UCGCTL4,
+		   I915_READ(GEN7_UCGCTL4) | GEN7_L3BANK2X_CLOCK_GATE_DISABLE);
 
 	I915_WRITE(MI_ARB_VLV, MI_ARB_DISPLAY_TRICKLE_FEED_DISABLE);