Provide dummy function pointers for the mock device in case we do hit
mmio during testing.

v2: Use ASSIGN_READ/WRITE_MMIO_FUNCS macros
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170412092143.3822-1-chris@chris-wilson.co.uk