drm: pl111: Absorb the external register header

The PL111 DRM driver is now the sole user of the external
CLCD registers header file, so let's absorb that into the
pl111_drm.h file and save the external include.

Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Cc: Russell King <linux@armlinux.org.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20200609200446.153209-3-linus.walleij@linaro.org