drm/i915: Allow userspace to specify ringsize on construction

No good reason why we must always use a static ringsize, so let
userspace select one during construction.

Link: https://github.com/intel/compute-runtime/pull/261


Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Steve Carbonari <steven.carbonari@intel.com>
Reviewed-by: Janusz Krzysztofik <janusz.krzysztofik@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200225192206.1107336-2-chris@chris-wilson.co.uk