drm: Add CONFIG_DRM_WERROR

Add kconfig to enable -Werror subsystem wide. This is useful for
development and CI to keep the subsystem warning free, while avoiding
issues outside of the subsystem that kernel wide CONFIG_WERROR=y might
hit.

v2: Don't depend on COMPILE_TEST

Reviewed-by: Hamza Mahfooz <hamza.mahfooz@amd.com> # v1
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Acked-by: Maxime Ripard <mripard@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/afe5ed943414f7ec3044c1547503b9941686a867.1709629403.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>