drm/tests: Add test for drm_framebuffer_lookup()

Add two KUnit test cases for the drm_framebuffer_lookup function, one
for the base case, that tests if the lookup finds the correct framebuffer
object and another that tests the lookup for an inexistent framebuffer.

Signed-off-by: Carlos Eduardo Gallo Filho <gcarlos@disroot.org>
Acked-by: Maxime Ripard <mripard@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20240911001559.28284-8-gcarlos@disroot.org


Signed-off-by: Maxime Ripard <mripard@kernel.org>