drm/tests: Stop using deprecated dev_private member on drm_framebuffer tests

drm/tests: Stop using deprecated dev_private member on drm_framebuffer tests

The dev_private member of drm_device is deprecated and its use should
be avoided. Stop using it by embedding the drm_device onto a mock struct.

The new mock struct allows to share variables and even further mocks
over the tests in a cleaner way than using dev_private void pointer.

Also start using drm_kunit_helper_alloc_drm_device() for allocating
the drm_device mock.

Signed-off-by: Carlos Eduardo Gallo Filho <gcarlos@disroot.org>
Acked-by: Maxime Ripard <mripard@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20240911001559.28284-2-gcarlos@disroot.org
Signed-off-by: Maxime Ripard <mripard@kernel.org>