drm/amdgpu: don't change mode in amdgpu_dm_connector_mode_valid()

Make amdgpu_dm_connector_mode_valid() duplicate the mode during the
test rather than modifying the passed mode. This is a preparation to
converting the mode_valid() callback of drm_connector to take a const
struct drm_display_mode argument.

Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.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-v2-2-4f9498a4c822@linaro.org


Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>