[DRIVERS] video: Introduce fb_get_smem() and fb_set_smem() API

For implementing "vscreen hard_bind" (i.e. hardware assisted virtual
screen capturing) we will need ability to switch frame buffer shared
memory start address in host frame buffer.

This patch introduces fb_get_smem() and fb_set_smem() frame buffer
APIs for this purpose.

Please note fb_set_smem() will not directly update fix screen info,
instead it will depend on fb_set_smem operation of host frame buffer
because the ability of switching frame buffer shared memory start
address is HW specific and all types of frame buffer HW will not have
this ability.

Signed-off-by: Anup Patel <anup@brainfault.org>