drm/i915: don't read LVDS regs at compute_config time

We may reach this point while the machine is still runtime suspended,
so we'll hit a WARN. The other encoders also don't touch registers at
this point, so instead of waking the machine up, write some code to
keep the register always at the same state, including after we runtime
suspend/resume.

Testcase: igt/pm_rpm
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=80463
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_lvds.c

@@ -51,6 +51,7 @@ struct intel_lvds_encoder {
 
 	bool is_dual_link;
 	u32 reg;
+	u32 a3_power;
 
 	struct intel_lvds_connector *attached_connector;
 };
@@ -170,8 +171,11 @@ static void intel_pre_enable_lvds(struct intel_encoder *encoder)
 
 	/* It would be nice to set 24 vs 18-bit mode (LVDS_A3_POWER_UP)
 	 * appropriately here, but we need to look more thoroughly into how
-	 * panels behave in the two modes.
+	 * panels behave in the two modes. For now, let's just maintain the
+	 * value we got from the BIOS.
 	 */
+	 temp &= ~LVDS_A3_POWER_MASK;
+	 temp |= lvds_encoder->a3_power;
 
 	/* Set the dithering flag on LVDS as needed, note that there is no
 	 * special lvds dither control bit on pch-split platforms, dithering is
@@ -269,7 +273,6 @@ static bool intel_lvds_compute_config(struct intel_encoder *intel_encoder,
 				      struct intel_crtc_config *pipe_config)
 {
 	struct drm_device *dev = intel_encoder->base.dev;
-	struct drm_i915_private *dev_priv = dev->dev_private;
 	struct intel_lvds_encoder *lvds_encoder =
 		to_lvds_encoder(&intel_encoder->base);
 	struct intel_connector *intel_connector =
@@ -284,8 +287,7 @@ static bool intel_lvds_compute_config(struct intel_encoder *intel_encoder,
 		return false;
 	}
 
-	if ((I915_READ(lvds_encoder->reg) & LVDS_A3_POWER_MASK) ==
-	    LVDS_A3_POWER_UP)
+	if (lvds_encoder->a3_power == LVDS_A3_POWER_UP)
 		lvds_bpp = 8*3;
 	else
 		lvds_bpp = 6*3;
@@ -1086,6 +1088,9 @@ out:
 	DRM_DEBUG_KMS("detected %s-link lvds configuration\n",
 		      lvds_encoder->is_dual_link ? "dual" : "single");
 
+	lvds_encoder->a3_power = I915_READ(lvds_encoder->reg) &
+				 LVDS_A3_POWER_MASK;
+
 	/*
 	 * Unlock registers and just
 	 * leave them unlocked