drm/i915: make more headers self-contained

The headers in the gem/selftests/, gt/selftests, gvt/, selftests/
directories have never been compile-tested, but it would be possible
to make them self-contained.

This commit only addresses missing <linux/types.h> and forward
struct declarations.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20191108094142.25942-1-yamada.masahiro@socionext.com