drm/i915/gtt: set err to -ENOMEM on memory allocation failure

Currently when the allocation of ppgtt->work fails the error return
path via err_free returns an uninitialized value in err. Fix this
by setting err to the appropriate error return of -ENOMEM.

Addresses-Coverity: ("Uninitialized scalar variable")
Fixes: d3622099 ("drm/i915/gtt: Always acquire struct_mutex for gen6_ppgtt_cleanup")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190524212627.24256-1-colin.king@canonical.com