drm: add drm_exec selftests v4

Exercise at least all driver facing functions of this new component.

v2: add array test as well
v3: some kunit cleanups
v4: more tests and cleanups

Signed-off-by: Christian König <christian.koenig@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230711133122.3710-3-christian.koenig@amd.com