rpisense-fb: Add explicit fb_deferred_io_mmap hook

As of commit [1], introduced in 5.18, fbdev drivers that use
deferred IO and need mmap support must include an explicit fb_mmap
pointer to the fb_deferred_io_mmap.

Signed-off-by: Phil Elwell <phil@raspberrypi.com>

[1] 59055851 ("fbdev: Put mmap for deferred I/O into drivers")