drm/pl111: Fix module probe bug

Commit a30933c27602 ("drm/pl111: Support the Versatile Express")
Added a second module using the builtin_platform_driver() call,
which works fine as long as you do not try to build the PL111
driver as a module, because a module can only have one initcall
and cause the following build bug:

(...) multiple definition of `init_module' (...)

Reported-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Liviu Dudau <liviu.dudau@arm.com>
Cc: Pawel Moll <pawel.moll@arm.com>
Cc: Eric Anholt <eric@anholt.net>
Cc: Robin Murphy <robin.murphy@arm.com>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Fixes: a30933c27602 ("drm/pl111: Support the Versatile Express")
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20180503140431.5798-1-linus.walleij@linaro.org