drm/i915: Drop the misleading cast to the wrong user pointer type

The exec_list is of type drm_i915_gem_exec_object2 and so casting it to
a drm_i915_gem_relocation_entry is very confusing!

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>