drm/stm: Allow build with COMPILE_TEST=y

Allow stm to be built with COMPILE_TEST=y for greater
coverage. Builds fine on x86/x86_64 at least.

Cc: Yannick Fertre <yannick.fertre@foss.st.com>
Cc: Raphael Gallais-Pou <raphael.gallais-pou@foss.st.com>
Cc: Philippe Cornu <philippe.cornu@foss.st.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240408170426.9285-22-ville.syrjala@linux.intel.com
Acked-by: Raphaël Gallais-Pou <raphael.gallais-pou@foss.st.com>