drm/i915: Do not read GAMMA_MODE register

Implement Daniel Stone's recommendation to not read registers to infer
the hardware's state.

v2: Read GAMMA_MODE register value at init (Matt Roper's comment)

v3: Read GAMMA_MODE register in intel_modeset_readout_hw_state along
    with other registers (Matt Roper's comment).

v4: Mask GAMMA_MODE register with interesting bits when reading

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1458125837-2576-3-git-send-email-lionel.g.landwerlin@intel.com

drivers/gpu/drm/i915/intel_color.c

@@ -121,6 +121,8 @@ static void haswell_load_luts(struct drm_crtc *crtc)
 	struct drm_device *dev = crtc->dev;
 	struct drm_i915_private *dev_priv = dev->dev_private;
 	struct intel_crtc *intel_crtc = to_intel_crtc(crtc);
+	struct intel_crtc_state *intel_crtc_state =
+		to_intel_crtc_state(crtc->state);
 	bool reenable_ips = false;
 
 	/*
@@ -128,11 +130,12 @@ static void haswell_load_luts(struct drm_crtc *crtc)
 	 * GAMMA_MODE is configured for split gamma and IPS_CTL has IPS enabled.
 	 */
 	if (IS_HASWELL(dev) && intel_crtc->config->ips_enabled &&
-	    ((I915_READ(GAMMA_MODE(intel_crtc->pipe)) & GAMMA_MODE_MODE_MASK) ==
-	     GAMMA_MODE_MODE_SPLIT)) {
+	    (intel_crtc_state->gamma_mode == GAMMA_MODE_MODE_SPLIT)) {
 		hsw_disable_ips(intel_crtc);
 		reenable_ips = true;
 	}
+
+	intel_crtc_state->gamma_mode = GAMMA_MODE_MODE_8BIT;
 	I915_WRITE(GAMMA_MODE(intel_crtc->pipe), GAMMA_MODE_MODE_8BIT);
 
 	i9xx_load_luts(crtc);

drivers/gpu/drm/i915/intel_display.c

@@ -9961,6 +9961,9 @@ static bool haswell_get_pipe_config(struct intel_crtc *crtc,
 
 	intel_get_pipe_src_size(crtc, pipe_config);
 
+	pipe_config->gamma_mode =
+		I915_READ(GAMMA_MODE(crtc->pipe)) & GAMMA_MODE_MODE_MASK;
+
 	if (INTEL_INFO(dev)->gen >= 9) {
 		skl_init_scalers(dev, crtc, pipe_config);
 	}

drivers/gpu/drm/i915/intel_drv.h

@@ -578,6 +578,9 @@ struct intel_crtc_state {
 		 */
 		bool need_postvbl_update;
 	} wm;
+
+	/* Gamma mode programmed on the pipe */
+	uint32_t gamma_mode;
 };
 
 struct vlv_wm_state {