compositor-drm: clear gbm_surface pointer after destroying the GBM surface

Since commit ee1d968e ("compositor-drm: Fall back if GBM surface fails with
modifiers"), drm_output_init_egl requires output->gbm_surface to be NULL, or
gbm_surface_create will not be called if HAVE_GBM_MODIFIERS is enabled but no
modifiers are supported by the plane. This could happen if _init_egl is called
after drm_ouptut_fini_egl drom drm_output_switch_mode.

Add an assert to guarantee the requirement and clears the gbm_surface pointer
after the surface is destroyed.

Signed-off-by: Philipp Zabel <p.pzabel@pengutronix.de>