drm/i915: add Makefile magic for testing headers are self-contained

The below commits added dummy files to test that certain headers are
self-contained, i.e. compilable as standalone units:

39e2f501 ("drm/i915: Split struct intel_context definition to its own header")
3a891a62 ("drm/i915: Move intel_engine_mask_t around for use by i915_request_types.h")
8b74594a

 ("drm/i915: Split out i915_priolist_types into its own header")

The idea is fine, but the implementation is a bit tedious and
inflexible, and does not really scale well.

Implement the same in make using autogenerated dummy sources to include
the headers.

v2 by Chris:
- Use patsubst
- Add .gitignore
- Add clean-files for generated dummy sources

v3 by Jani:
- Fix make clean
- Add the tests to i915-y instead of extra-y

v4 by Jani:
- quiet_cmd whitespace fix

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.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/20190403135236.8398-1-jani.nikula@intel.com