drm/i915/selftests: Increase size for mock ringbuffer

We don't actually emit any commands into the ringbuffer, so we set it
very small. However, an upcoming change centralises the wait-for-space
into i915_gem_request_alloc() and that imposes a minimum size upon all
ringbuffers (mock or real) of MIN_SPACE_FOR_ADD_REQUEST. Grow the
mock ringbuffer such that we allocate a single page for the struct+buffer,
satisfying the new condition without wasting too much space.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171115151204.8105-1-chris@chris-wilson.co.uk