drm/i915: Add missing ring_mask to Pineview

It appears that we never told Pineview it has a RENDER_RING. This was
all fine until we started using the ring_mask for determining all the
available rings to initialise for legacy ringbuffer submission in commit
88d2ba2e ("drm/i915: Unify engine init loop"). Though really it is a
latent bug since the ring_mask inception in commit 73ae478c
("drm/i915: Replace has_bsd/blt/vebox with a mask").

To prevent similar mishaps in future, add a WARN_ON() if we find
ourselves with a device without any rings.

Fixes: 73ae478c ("drm/i915: Replace has_bsd/blt/vebox with a mask")
Fixes: 88d2ba2e ("drm/i915: Unify engine init loop")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Ben Widawsky <ben@bwidawsk.net>
Link: http://patchwork.freedesktop.org/patch/msgid/1469749535-2382-1-git-send-email-chris@chris-wilson.co.uk


Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: drm-intel-fixes@lists.freedesktop.org