drm: have config DRM_WERROR depend on !WERROR

If WERROR is already enabled, there's no point in enabling DRM_WERROR or
asking users about it.

Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Closes: https://lore.kernel.org/r/CAHk-=whxT8D_0j=bjtrvj-O=VEOjn6GW8GK4j2V+BiDUntZKAQ@mail.gmail.com
Fixes: f89632a9

 ("drm: Add CONFIG_DRM_WERROR")
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240516083343.1375687-1-jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>