Add section for drm/i915 uAPI and pull in i915_drm.h.

Suggested-by: Daniel Vetter
Signed-off-by: Matthew Auld
Cc: Joonas Lahtinen
Cc: Jordan Justen
Cc: Daniel Vetter
Cc: Kenneth Graunke
Cc: Jason Ekstrand
Cc: Dave Airlie
Cc: dri-devel@lists.freedesktop.org
Cc: mesa-dev@lists.freedesktop.org
Reviewed-by: Daniel Vetter
Link: https://patchwork.freedesktop.org/patch/msgid/20210419105741.27844-2-matthew.auld@intel.com