drm/i915: Program FW_BLC_SELF on 915G as well

According to Bspec FW_BLC_SELF exists on 915G also. Let's program it.
The only open question is whether there's is a memory self-refresh
enable bit somewhere as well. For 945G/GM it's in FW_BLC_SELF, for
915GM it's in INSTPM. For 915G I can't find one in the docs. Let's drop
a FIXME about this, in case someone with the hardware is ever bored
enough to look for it.

Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1469804222-12650-2-git-send-email-ville.syrjala@linux.intel.comReviewed-by: Chris Wilson <chris@chris-wilson.co.uk>

drivers/gpu/drm/i915/intel_pm.c

@@ -340,6 +340,11 @@ void intel_set_memory_cxsr(struct drm_i915_private *dev_priv, bool enable)
 		I915_WRITE(FW_BLC_SELF, val);
 		POSTING_READ(FW_BLC_SELF);
 	} else if (IS_I915GM(dev)) {
+		/*
+		 * FIXME can't find a bit like this for 915G, and
+		 * and yet it does have the related watermark in
+		 * FW_BLC_SELF. What's going on?
+		 */
 		val = enable ? _MASKED_BIT_ENABLE(INSTPM_SELF_EN) :
 			       _MASKED_BIT_DISABLE(INSTPM_SELF_EN);
 		I915_WRITE(INSTPM, val);
@@ -1621,7 +1626,7 @@ static void i9xx_update_wm(struct drm_crtc *unused_crtc)
 		if (IS_I945G(dev) || IS_I945GM(dev))
 			I915_WRITE(FW_BLC_SELF,
 				   FW_BLC_SELF_FIFO_MASK | (srwm & 0xff));
-		else if (IS_I915GM(dev))
+		else
 			I915_WRITE(FW_BLC_SELF, srwm & 0x3f);
 	}