drm/radeon: fix AGP dependency

When AGP is compiled as module radeon must be compiled as module as
well.
Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/Kconfig

@@ -225,6 +225,7 @@ source "drivers/gpu/drm/arm/Kconfig"
 config DRM_RADEON
 	tristate "ATI Radeon"
 	depends on DRM && PCI && MMU
+	depends on AGP || !AGP
 	select FW_LOADER
         select DRM_KMS_HELPER
         select DRM_TTM