drm: drm_gem, check kzalloc retval

Check kzalloc retval against NULL in drm_gem_object_alloc and bail out
appropriately.

While at it merge the fail paths and jump to them by gotos at the end
of the function.

Signed-off-by: Jiri Slaby <jirislaby@gmail.com>
Signed-off-by: Dave Airlie <airlied@linux.ie>