drm/vmwgfx: Remove STDU logic from generic mode_valid function

STDU has its own mode_valid function now so this logic can be removed from
the generic version.

Fixes: 935f7950

 ("drm/vmwgfx: Refactor drm connector probing for display modes")

Signed-off-by: Ian Forbes <ian.forbes@broadcom.com>
Signed-off-by: Zack Rusin <zack.rusin@broadcom.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240521184720.767-4-ian.forbes@broadcom.com