drm/omap: Allow build with COMPILE_TEST=y

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

Cc: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240408170426.9285-16-ville.syrjala@linux.intel.com
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>