gma500: Fix mmap frambuffer

It cannot map correctly if page fault begins from a intermediate address.

[The driver prefaults the mapping, so we need to work from the correct
 base address not the faulting address otherwise the map appears offset by
 the fault offset]

Signed-off-by: Yoichi Yuasa <yuasa@linux-mips.org>
Signed-off-by: Alan Cox <alan@linux.intel.com>
Cc: stable@kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>