drm/ingenic: prepare ingenic drm for later addition of JZ4780

This changes the way the regmap is allocated to prepare for the
later addition of the JZ4780 which has more registers and bits
than the others.

Therefore we make the regmap as big as the reg property in
the device tree tells.

Suggested-by: Paul Cercueil <paul@crapouillou.net>
Signed-off-by: H. Nikolaus Schaller <hns@goldelico.com>
Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Link: https://patchwork.freedesktop.org/patch/msgid/ee65e70e91268b3963f8a6581c8aa6c3b643c53e.1638470392.git.hns@goldelico.com