drm/i915: Track HAS_RPS alongside HAS_RC6 in the device info

For consistency (and elegance!), add intel_device_info.has_rps.
The immediate boon is that RPS support is now emitted along the other
capabilities in the debug log and after errors.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Sagar Arun Kamble <sagar.a.kamble@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190419134836.5626-1-chris@chris-wilson.co.uk