drm/plane_helper: Print actual/expected values on failure

Currently the values are printed with debug log level.
Adjust the log level and link the output with the test by using kunit_err.

Example output:
foo: dst: 20x20+10+10, expected: 10x10+0+0
foo: EXPECTATION FAILED at drivers/gpu/drm/tests/drm_plane_helper_test.c:85

Signed-off-by: Michał Winiarski <michal.winiarski@intel.com>
Reviewed-by: Maíra Canal <mairacanal@riseup.net>
Signed-off-by: Maíra Canal <mairacanal@riseup.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20221020082135.779872-1-michal.winiarski@intel.com