drm/i915: Share the computation of ring size for RING_CTL register

Since both legacy and execlists want to populate the RING_CTL register,
share the computation of the right bits for the ring->size. We can then
stop masking errors and explicitly forbid them during creation!

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161004201132.21801-1-chris@chris-wilson.co.uk