drm/ci: Add jobs to validate devicetrees

Add jobs to run dt_binding_check and dtbs_check. If warnings are seen,
exit with a non-zero error code while configuring them as warning in
the GitLab CI pipeline.

Signed-off-by: Vignesh Raman <vignesh.raman@collabora.com>
Acked-by: Helen Koike <helen.fornazier@gmail.com>
Reviewed-by: Maxime Ripard <mripard@kernel.org>
Acked-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
Link: https://lore.kernel.org/r/20250623085033.39680-2-vignesh.raman@collabora.com


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