This allows to mimick i915's requirement of failing all commits
which don't have the primary plane enabled.
We could expand our test matrix to test with the param *and* without
it, but since enabling the param just makes the atomic check more
strict there's no real benefit of doing that. Enabling the param
can only make tests fail more often, not the other way around.
The previous mock library assumed the DRM client would always set all
properties in the atomic request. However, if a previous test-only commit
has set a property, there's no need to set it again.
Accomodate for this in libdrm_mock. This will be useful when we'll skip
setting properties that haven't changed in libliftoff.
References: https://github.com/emersion/libliftoff/issues/37
When testing the library, the libdrm dependency is swapped with a mock libdrm.
The test sets up the mock libdrm's internal state and then runs the test.
