drm/i915: use kernel-doc -Werror when CONFIG_DRM_I915_WERROR=y

With CONFIG_DRM_I915_WERROR=y, we enable kernel-doc check for both
objects and headers. Now that the kernel-doc warnings have been fixed,
also enable kernel-doc -Werror to fail the build on kernel-doc warnings.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/68ffcad0e6ff2b6cd70c6df28822f967898ce197.1683041799.git.jani.nikula@intel.com