drm/i915: Don't touch NULL sg on i915_gem_object_get_pages_gtt() error

drm/i915: Don't touch NULL sg on i915_gem_object_get_pages_gtt() error

On the DMA mapping error path, sg may be NULL (it has already been
marked as the last scatterlist entry), and we should avoid dereferencing
it again.

Reported-by: Dan Carpenter <[email protected]>
Fixes: e227330223a7 ("drm/i915: avoid leaking DMA mappings")
Signed-off-by: Chris Wilson <[email protected]>
Cc: Imre Deak <[email protected]>
Cc: [email protected]
Link: http://patchwork.freedesktop.org/patch/msgid/[email protected]
Reviewed-by: Matthew Auld <[email protected]>