drivers/i915: Fix unnoticed failure of init_ring_common()

This commit add check for return value of init_ring_common() in the
init_render_ring(). Now, when failure is detected the error code is
propagated to the caller instead of being ignored.

Signed-off-by: Konrad Zapalowicz <bergo.torino@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>