Passing NULL ctx to request_alloc would lead to null-ptr-deref.

v2: Let's not replace the comment with a BUG_ON
Signed-off-by: NMichał Winiarski <michal.winiarski@intel.com>
Reviewed-by: NChris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20170523102400.9614-1-michal.winiarski@intel.comSigned-off-by: NChris Wilson <chris@chris-wilson.co.uk>