To avoid having to create all the device and driver scaffolding we
just manually create and destroy a devres_group.

v2: Rebased

v3: use devres_open/release_group so we can use devm without real
hacks in the driver core or having to create an entire fake bus for
testing drivers. Might want to extract this into helpers eventually,
maybe as a mock_drm_dev_alloc or test_drm_dev_alloc.

v4:
- Fix IS_ERR handling (Matt)
- Delete surplus put_device() in mock_device_release (intel-gfx-ci)

v5:
- do not switch to device_add - it breaks runtime pm in the tests and
  with the devres_group_add/release no longer needed for automatic
  cleanup (CI). Update commit message to match.
- print correct error in pr_err (Matt)

v6: Remove now unused err variable (CI).

v7: More warning fixes ...
Acked-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Matthew Auld <matthew.william.auld@gmail.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com> (v3)
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Matthew Auld <matthew.william.auld@gmail.com> (v4)
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200919134032.2488403-1-daniel.vetter@ffwll.ch