rpisense-fb: Add explicit fb_deferred_io_mmap hook

commit  0dad8f4ad7c4b06a4aed86222f661aff474511cf from
https://github.com/raspberrypi/linux.git

 rpi-6.1.y

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")
Signed-off-by: Meng Li <Meng.Li@windriver.com>