drm/mipi-dsi: Add dev_is_mipi_dsi function

This will be especially useful for generic panels (like panel-simple)
which can take different code path depending on if they are MIPI-DSI
devices or platform devices.

Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Tested-by: Francesco Dolcini <francesco.dolcini@toradex.com> # Toradex Colibri iMX6
Link: https://lore.kernel.org/r/20250626-drm-panel-simple-fixes-v2-1-5afcaa608bdc@kernel.org


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