drm/ingenic: Move register definitions to ingenic-drm.h

Move the register definitions to ingenic-drm.h, to keep
ingenic-drm-drv.c tidy.

v2: Fix SPDX license tag
v3: No change

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200716163846.174790-7-paul@crapouillou.net