drm/i915: Reset glk degamma index after programming/readout

Just for some extra consistency let's reset the glk degamma LUT
index back to 0 after we're dong trawling the LUT.
Reviewed-by: NUma Shankar <uma.shankar@intel.com>
Signed-off-by: NVille Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200925131656.10022-6-ville.syrjala@linux.intel.com

drivers/gpu/drm/i915/display/intel_color.c

@@ -818,12 +818,14 @@ static void glk_load_degamma_lut(const struct intel_crtc_state *crtc_state)
 		 * as compared to just 16 to achieve this.
 		 */
 		intel_de_write(dev_priv, PRE_CSC_GAMC_DATA(pipe),
-		               lut[i].green);
+			       lut[i].green);
 	}
 
 	/* Clamp values > 1.0. */
 	while (i++ < 35)
 		intel_de_write(dev_priv, PRE_CSC_GAMC_DATA(pipe), 1 << 16);
+
+	intel_de_write(dev_priv, PRE_CSC_GAMC_INDEX(pipe), 0);
 }
 
 static void glk_load_degamma_lut_linear(const struct intel_crtc_state *crtc_state)
@@ -851,6 +853,8 @@ static void glk_load_degamma_lut_linear(const struct intel_crtc_state *crtc_stat
 	/* Clamp values > 1.0. */
 	while (i++ < 35)
 		intel_de_write(dev_priv, PRE_CSC_GAMC_DATA(pipe), 1 << 16);
+
+	intel_de_write(dev_priv, PRE_CSC_GAMC_INDEX(pipe), 0);
 }
 
 static void glk_load_luts(const struct intel_crtc_state *crtc_state)