drm/pl111: fix warnings without CONFIG_ARM_AMBA

The driver is written in a way to enable compile-testing without CONFIG_ARM_AMBA,
but it just causes needless warnings:

drivers/gpu/drm/pl111/pl111_drv.c:149:26: error: 'pl111_drm_driver' defined but not used [-Werror=unused-variable]
drivers/gpu/drm/pl111/pl111_drv.c:81:12: error: 'pl111_modeset_init' defined but not used [-Werror=unused-function]

This unhides the probe/remove functions again and just leaves the driver
object as unused when CONFIG_ARM_AMBA is disabled, with a __maybe_unused
annotation to shut up the warning.

Fixes: bed41005 ("drm/pl111: Initial drm/kms driver for pl111")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Eric Anholt <eric@anholt.net>
Link: http://patchwork.freedesktop.org/patch/msgid/20170524155020.1777369-1-arnd@arndb.de