drm/i915: Introduce HAS_FPGA_DBG_UNCLAIMED()

Let's introduce one more of those orthogonal feature macros. This should
hopefully make the code more readable and make things easier for new platform
enabling.

This time, HAS_FPGA_DBG_UNCLAIMED() is true for platforms that have bit
31 of FPGA_DBG able to signal unclaimed writes.

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>