drm/i915: Initialize err in remap_io_sg()

If the do while loop breaks in 'if (!sg_dma_len(sgl))' in the first
iteration, err is uninitialized causing a wrong call to zap_vma_ptes().

But that is impossible to happen as a scatterlist must have at least
one valid segment.
Anyways to avoid more reports from static checkers initializing ret
here.

Fixes: b12d691e ("i915: fix remap_io_sg to verify the pgprot")
Reviewed-by: Christoph Hellwig <hch@lst.de>
Cc: Christoph Hellwig <hch@lst.de>
Signed-off-by: James Ausmus <james.ausmus@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210517202117.179303-1-jose.souza@intel.com