drm/i915: Clear the existing watermarks for g4x when modifying the cursor sr

In a couple of places we attempt to adjust the existing watermark
registers to update them for the new cursor watermarks. This goes
horribly wrong as instead of clearing the cursor bits prior to or'ing in
the new values, we clear the rest of the register with the result that
the watermark registers contain bogus values.

References: https://bugs.freedesktop.org/show_bug.cgi?id=47034
Signed-off-by: Chris Wilson <[e-mail]>
Reviewed-by: Paulo Zanoni <[e-mail]>
Signed-off-by: Daniel Vetter <[e-mail]>

drivers/gpu/drm/i915/intel_pm.c

···
 		   (planeb_wm << DSPFW_PLANEB_SHIFT) |
 		   planea_wm);
 	I915_WRITE(DSPFW2,
-		   (I915_READ(DSPFW2) & DSPFW_CURSORA_MASK) |
+		   (I915_READ(DSPFW2) & ~DSPFW_CURSORA_MASK) |
 		   (cursora_wm << DSPFW_CURSORA_SHIFT));
 	I915_WRITE(DSPFW3,
-		   (I915_READ(DSPFW3) | (cursor_sr << DSPFW_CURSOR_SR_SHIFT)));
+		   (I915_READ(DSPFW3) & ~DSPFW_CURSOR_SR_MASK) |
+		   (cursor_sr << DSPFW_CURSOR_SR_SHIFT));
 }
 
 static void g4x_update_wm(struct drm_device *dev)
···
 		   (planeb_wm << DSPFW_PLANEB_SHIFT) |
 		   planea_wm);
 	I915_WRITE(DSPFW2,
-		   (I915_READ(DSPFW2) & DSPFW_CURSORA_MASK) |
+		   (I915_READ(DSPFW2) & ~DSPFW_CURSORA_MASK) |
 		   (cursora_wm << DSPFW_CURSORA_SHIFT));
 	/* HPLL off in SR has some issues on G4x... disable it */
 	I915_WRITE(DSPFW3,
-		   (I915_READ(DSPFW3) & ~DSPFW_HPLL_SR_EN) |
+		   (I915_READ(DSPFW3) & ~(DSPFW_HPLL_SR_EN | DSPFW_CURSOR_SR_MASK)) |
 		   (cursor_sr << DSPFW_CURSOR_SR_SHIFT));
 }