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>

drivers/gpu/drm/gma500/framebuffer.c

@@ -125,7 +125,7 @@ static int psbfb_vm_fault(struct vm_area_struct *vma, struct vm_fault *vmf)
 	unsigned long phys_addr = (unsigned long)dev_priv->stolen_base;
 
 	page_num = (vma->vm_end - vma->vm_start) >> PAGE_SHIFT;
-	address = (unsigned long)vmf->virtual_address;
+	address = (unsigned long)vmf->virtual_address - (vmf->pgoff << PAGE_SHIFT);
 
 	vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot);