tools headers UAPI: Sync drm/i915_drm.h with the kernel sources

To pick the changes in:

  b5b6f6a6

 ("drm/i915/gem: Drop legacy execbuffer support (v2)")

That don't result in any change in tooling as this is just adding a
comment.

Only silences this perf build warning:

  Warning: Kernel ABI header at 'tools/include/uapi/drm/i915_drm.h' differs from latest version at 'include/uapi/drm/i915_drm.h'
  diff -u tools/include/uapi/drm/i915_drm.h include/uapi/drm/i915_drm.h

Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Jason Ekstrand <jason@jlekstrand.net>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>