drm/i915/dsi: Do not clear DPOUNIT_CLOCK_GATE_DISABLE from vlv_init_display_clock_gating

On my Cherrytrail CUBE iwork8 Air tablet PIPE-A would get stuck on loading
i915 at boot 1 out of every 3 boots, resulting in a non functional LCD.
Once the i915 driver has successfully loaded, the panel can be disabled /
enabled without hitting this issue.

The getting stuck is caused by vlv_init_display_clock_gating() clearing
the DPOUNIT_CLOCK_GATE_DISABLE bit in DSPCLK_GATE_D when called from
chv_pipe_power_well_ops.enable() on driver load, while a pipe is enabled
driving the DSI LCD by the BIOS.

Clearing this bit while DSI is in use is a known issue and
intel_dsi_pre_enable() / intel_dsi_post_disable() already set / clear it
as appropriate.

This commit modifies vlv_init_display_clock_gating() to leave the
DPOUNIT_CLOCK_GATE_DISABLE bit alone fixing the pipe getting stuck.

Changes in v2:
-Replace PIPE-A with "a pipe" or "the pipe" in the commit msg and
comment

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=97330
Cc: stable@vger.kernel.org
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161202142904.25613-1-hdegoede@redhat.com
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>

drivers/gpu/drm/i915/intel_runtime_pm.c

@@ -1099,7 +1099,18 @@ static bool vlv_power_well_enabled(struct drm_i915_private *dev_priv,
 
 static void vlv_init_display_clock_gating(struct drm_i915_private *dev_priv)
 {
-	I915_WRITE(DSPCLK_GATE_D, VRHUNIT_CLOCK_GATE_DISABLE);
+	u32 val;
+
+	/*
+	 * On driver load, a pipe may be active and driving a DSI display.
+	 * Preserve DPOUNIT_CLOCK_GATE_DISABLE to avoid the pipe getting stuck
+	 * (and never recovering) in this case. intel_dsi_post_disable() will
+	 * clear it when we turn off the display.
+	 */
+	val = I915_READ(DSPCLK_GATE_D);
+	val &= DPOUNIT_CLOCK_GATE_DISABLE;
+	val |= VRHUNIT_CLOCK_GATE_DISABLE;
+	I915_WRITE(DSPCLK_GATE_D, val);
 
 	/*
 	 * Disable trickle feed and enable pnd deadline calculation