drm/vmwgfx: Refactor drm connector probing for display modes

Implement drm_connector_helper_funcs.mode_valid and .get_modes,
replacing custom drm_connector_funcs.fill_modes code with
drm_helper_probe_single_connector_modes; for STDU, LDU & SOU
display units.

Signed-off-by: Martin Krastev <martin.krastev@broadcom.com>
Reviewed-by: Zack Rusin <zack.rusin@broadcom.com>
Signed-off-by: Zack Rusin <zack.rusin@broadcom.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240126200804.732454-2-zack.rusin@broadcom.com