compositor-drm: clear gbm_surface pointer after destroying the GBM surface
I think there is a possible use-after-free issue with output->gbm_surface if HAVE_GBM_MODIFIERS is enabled but the plane does not support any modifiers. If drm_output_fini_egl destroys the gbm_surface but does not clear the pointer, the following drm_output_ini_egl (for example called from drm_output_switch_mode) will not create a GBM surface because the pointer is not NULL.