drm/ingenic: Fix MODULE_LICENSE() string

The previous "GPL v2" string is deprecated. For more info, see commit
bf7fbeea ("module: Cure the MODULE_LICENSE "GPL" vs. "GPL v2" bogosity")

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20220708205406.96473-3-paul@crapouillou.net