drm/rockchip: rgb: update SPDX license identifier

Update SPDX License Identifier from GPL-2.0+ to GPL-2.0
and drop some GPL text.
This fixes a mismatch between the existing SPDX headers and GPL
boilerplate text.

Fixes: 1f0f0151 ("Add support for Rockchip Soc RGB output interface")
Cc: stable@vger.kernel.org
Reported-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Sandy Huang <hjc@rock-chips.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/1548238479-171491-1-git-send-email-hjc@rock-chips.com