drm/i915/bxt: Disable DSI PLL for BXT

This patch adds two new functions:
- disable_dsi_pll.
  BXT DSI disable sequence and registers are
  different from previous platforms.
- intel_disable_dsi_pll
  wrapper function to re-use the same code for
  multiple platforms. It checks platform type and
  calls appropriate core pll disable function.

v2: Fixed Jani's review comments.

v3: Rebased on latest drm-nightly branch.

Signed-off-by: Shashank Sharma <shashank.sharma@intel.com>
Signed-off-by: Uma Shankar <uma.shankar@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
diff --git a/drivers/gpu/drm/i915/intel_dsi.c b/drivers/gpu/drm/i915/intel_dsi.c
index f2f8196..50159a9 100644
--- a/drivers/gpu/drm/i915/intel_dsi.c
+++ b/drivers/gpu/drm/i915/intel_dsi.c
@@ -557,7 +557,7 @@
 		usleep_range(2000, 2500);
 	}
 
-	vlv_disable_dsi_pll(encoder);
+	intel_disable_dsi_pll(encoder);
 }
 
 static void intel_dsi_post_disable(struct intel_encoder *encoder)