compositor-drm: Don't set linear modifier when not supported

This will cause gbm_surface_create_with_modifiers to fail on drivers
where modifiers are not yet supported (e.g. amdgpu). We need to make
sure we only end up using gbm_surface_create in this case.

This fixes the remoting plugin on these drivers.

Signed-off-by: Scott Anderson <scott.anderson@collabora.com>