When the caller wants to perform a modeset, they will need to pass
DRM_MODE_ATOMIC_ALLOW_MODESET in the atomic commit flags, otherwise the driver
will fail the commit. libliftoff also needs to provide these flags in its
test-only commits.
This fixes a misunderstanding of the DRM API. Only a regular commit applies
properties.
Fixes: b16078769e ("test/libdrm_mock: a test-only commit applies properties")