drm/i915: reimplement header test feature

I implemented a small build rule in drivers/gpu/drm/i915/Makefile
without relying on the special header-test-y syntax that was removed in
commit fcbb8461 ("kbuild: remove header compile test").

I excluded some headers from the test coverage. I hope somebody
intrested can take a closer look at them.

Dummy subdir Makefiles can be removed altogether as single target build
use case is now covered by commit 394053f4 ("kbuild: make single
targets work more correctly").

v2 by Jani:
- add selftests/i915_perf_selftests.h to no-header-test
- add .gitignore for *.hdrtest

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
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/20191219155652.2666-3-jani.nikula@intel.com