drm/simple-kms: Standardize arguments for callbacks

Passing the wrong type feels icky, everywhere else we use the pipe as
the first parameter. Spotted while discussing patches with Thomas
Zimmermann.

v2: Make xen compile correctly

Acked-By: Thomas Zimmermann <tzimmermann@suse.de> (v1)
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Noralf Trønnes <noralf@tronnes.org>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Eric Anholt <eric@anholt.net>
Cc: Emil Velikov <emil.velikov@collabora.com>
Cc: virtualization@lists.linux-foundation.org
Cc: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191023101256.20509-1-daniel.vetter@ffwll.ch