drm/i915/bxt: Remove DSP CLK_GATE programming for BXT

DSP CLK_GATE registers are specific to BYT and CHT.
Avoid programming the same for BXT platform.

v2: Rebased on latest drm nightly branch.

v3: Fixed Jani's review comments
Signed-off-by: NUma Shankar <uma.shankar@intel.com>
Signed-off-by: NJani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1455796166-13052-1-git-send-email-jani.nikula@intel.com

drivers/gpu/drm/i915/intel_dsi.c

@@ -634,7 +634,6 @@ static void intel_dsi_post_disable(struct intel_encoder *encoder)
 {
 	struct drm_i915_private *dev_priv = encoder->base.dev->dev_private;
 	struct intel_dsi *intel_dsi = enc_to_intel_dsi(&encoder->base);
-	u32 val;
 
 	DRM_DEBUG_KMS("\n");
 
@@ -642,9 +641,13 @@ static void intel_dsi_post_disable(struct intel_encoder *encoder)
 
 	intel_dsi_clear_device_ready(encoder);
 
-	val = I915_READ(DSPCLK_GATE_D);
-	val &= ~DPOUNIT_CLOCK_GATE_DISABLE;
-	I915_WRITE(DSPCLK_GATE_D, val);
+	if (!IS_BROXTON(dev_priv)) {
+		u32 val;
+
+		val = I915_READ(DSPCLK_GATE_D);
+		val &= ~DPOUNIT_CLOCK_GATE_DISABLE;
+		I915_WRITE(DSPCLK_GATE_D, val);
+	}
 
 	drm_panel_unprepare(intel_dsi->panel);