drm/encoder_slave: make mode_valid accept const struct drm_display_mode

drm/encoder_slave: make mode_valid accept const struct drm_display_mode

The mode_valid() callbacks of drm_encoder, drm_crtc and drm_bridge
accept const struct drm_display_mode argument. Change the mode_valid
callback of drm_encoder_slave to also accept const argument.

Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Maxime Ripard <mripard@kernel.org>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241214-drm-connector-mode-valid-const-v2-1-4f9498a4c822@linaro.org
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>