drm/i915/csr: move CSR version macros to intel_csr.h

Reduce clutter from i915_drv.h.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/8222df3f559b056387b5c7e6e04a878cbf8b4e2e.1556809195.git.jani.nikula@intel.com