drm/vmwgfx: check kmalloc return value

drm/vmwgfx: check kmalloc return value

srf->sizes has been allocated and checked a few lines above; fix up
the copy-pasto so that we check srf->offsets.

Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1450178476-26284-8-git-send-email-boris.brezillon@free-electrons.com
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>