drm/i915: Fix user ptr check size in eb_relocate_vma()

Fix the sizeof(ptr) vs. sizeof(*ptr) typo.

Fixes: 2889caa9 ("drm/i915: Eliminate lots of iterations over the execobjects array")
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20170714151242.517-2-imre.deak@intel.com