acornfb: remove fb_mmap function

The driver's fb_mmap function is essentially the same as a generic fb_mmap
function.  Delete driver's function and use the generic one.

A difference is that generic function marks frame buffer memory as VM_IO |
VM_RESERVED.  The driver's function marks it as VM_IO only.

Signed-off-by: Krzysztof Helt <krzysztof.h1@wp.pl>
Cc: Russell King <rmk@arm.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>