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.comAcked-by: Raphaël Gallais-Pou <raphael.gallais-pou@foss.st.com>

drivers/gpu/drm/stm/Kconfig

 # SPDX-License-Identifier: GPL-2.0-only
 config DRM_STM
 	tristate "DRM Support for STMicroelectronics SoC Series"
-	depends on DRM && ARCH_STM32
+	depends on DRM && (ARCH_STM32 || COMPILE_TEST)
 	select DRM_KMS_HELPER
 	select DRM_GEM_DMA_HELPER
 	select DRM_PANEL_BRIDGE