drm/i915: make intel_guc_reg.h self-contained

Add the minimal includes/declarations to make the header self-contained,
and ensure it stays that way.

v2: also include i915_reg.h (Michal)

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/20190626144020.2155-11-jani.nikula@intel.com