drm/i915: enable kernel-doc warnings for CONFIG_DRM_I915_WERROR=y

Increase awareness of kernel-doc issues by enabling doc check locally
when CONFIG_DRM_I915_WERROR=y is enabled.

Once the warnings have been fixed, we can pass -Werror to kernel-doc
locally, and fail the build when there are kernel-doc warnings in i915.

v2: Don't check for KBUILD_EXTRA_WARN (Masahiro)

Cc: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Acked-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230403125710.3617230-1-jani.nikula@intel.com