Add COLOR_MODE_DISPLAY_BT2020 to graphics header.

Bug: 256551001
Test: build and flash; hidl-gen -Lexport-header -o $ANDROID_BUILD_TOP/system/core/include/system/graphics-base-v1.2.h
android.hardware.graphics.common@1.2

Change-Id: I989d1171c98f477f174ad092b74d455a062c9bad