HID: picolcd: do not reallocate memory on depth change

Reallocating memory in depth change does not work well if some
userspace application has mmapped() the framebuffer as that mapping
does not get adjusted (thus application continues to write to old
buffer).
In addition doing deferred_io_cleanup() and init() inside of set_par()
tends to deadlock with fbcon's flashing cursor.

Avoid all this by allocating a buffer that can hold 8bpp framebuffer
and just use 1/8 of it while running at 1bpp.

Signed-off-by: Bruno Prémont <bonbons@linux-vserver.org>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>