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
diff --git a/drivers/gpu/drm/i915/intel_drv.h b/drivers/gpu/drm/i915/intel_drv.h
index 5d0da22..d4945bb 100644
--- a/drivers/gpu/drm/i915/intel_drv.h
+++ b/drivers/gpu/drm/i915/intel_drv.h
@@ -578,6 +578,9 @@
 		 */
 		bool need_postvbl_update;
 	} wm;
+
+	/* Gamma mode programmed on the pipe */
+	uint32_t gamma_mode;
 };
 
 struct vlv_wm_state {