Rework function signature so tinydrm_device can be avoided.

Move definition to tinydrm-helpers.h so tinydrm.h can be deleted in a
later patch.
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190210131039.52664-6-noralf@tronnes.org