drm/doc: add section for driver uAPI

Add section for drm/i915 uAPI and pull in i915_drm.h.
Suggested-by: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Daniel Vetter <daniel.vetter@intel.com>
Cc: Kenneth Graunke <kenneth@whitecape.org>
Cc: Jason Ekstrand <jason@jlekstrand.net>
Cc: Dave Airlie <airlied@gmail.com>
Cc: dri-devel@lists.freedesktop.org
Cc: mesa-dev@lists.freedesktop.org
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210419105741.27844-2-matthew.auld@intel.com

Documentation/gpu/driver-uapi.rst

+===============
+DRM Driver uAPI
+===============
+
+drm/i915 uAPI
+=============
+
+.. kernel-doc:: include/uapi/drm/i915_drm.h

Documentation/gpu/index.rst

@@ -10,6 +10,7 @@ Linux GPU Driver Developer's Guide
    drm-kms
    drm-kms-helpers
    drm-uapi
+   driver-uapi
    drm-client
    drivers
    backlight