drm: ensure drm headers are self-contained and pass kernel-doc

drm: ensure drm headers are self-contained and pass kernel-doc

Ensure drm headers build, are self-contained, have header guards, and
have no kernel-doc warnings, when CONFIG_DRM_HEADER_TEST=y.

The mechanism follows similar patters used in i915, xe, and usr/include.

To cover include/drm, we need to recurse there using the top level
Kbuild and the new include/Kbuild files.

v4: check for CONFIG_WERROR in addition to CONFIG_DRM_WERROR

v3: adapt to upstream build changes

v2: make DRM_HEADER_TEST depend on DRM

Suggested-by: Daniel Vetter <daniel@ffwll.ch>
Cc: David Airlie <airlied@gmail.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Masahiro Yamada <masahiroy@kernel.org>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Simona Vetter <simona.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/d8ad1c6d707f38a55987f616cb9650aef30b84e1.1737556766.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>