drm/i915: Remove misleading comment in request_alloc

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: Michał Winiarski <michal.winiarski@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20170523102400.9614-1-michal.winiarski@intel.comSigned-off-by: Chris Wilson <chris@chris-wilson.co.uk>

drivers/gpu/drm/i915/i915_gem_request.c

@@ -538,9 +538,6 @@ submit_notify(struct i915_sw_fence *fence, enum i915_sw_fence_notify state)
  *
  * @engine: engine that we wish to issue the request on.
  * @ctx: context that the request will be associated with.
- *       This can be NULL if the request is not directly related to
- *       any specific user context, in which case this function will
- *       choose an appropriate context to use.
  *
  * Returns a pointer to the allocated request if successful,
  * or an error code if not.