drm/i915: Annotate user relocs with __user

Add the missing __user to the urelocs cast to fix the following sparse
warning:
i915_gem_execbuffer.c:1541:47: warning: cast removes address space of expression
i915_gem_execbuffer.c:1541:62: warning: incorrect type in argument 2 (different address spaces)
i915_gem_execbuffer.c:1541:62:    expected void const [noderef] <asn:1>*from
i915_gem_execbuffer.c:1541:62:    got char *

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Fixes: 2889caa9 ("drm/i915: Eliminate lots of iterations over the execobjects array")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170901165434.24636-1-ville.syrjala@linux.intel.com
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com> #irc