drm/vmwgfx: 3D disabled should not effect STDU memory limits

This limit became a hard cap starting with the change referenced below.
Surface creation on the device will fail if the requested size is larger
than this limit so altering the value arbitrarily will expose modes that
are too large for the device's hard limits.

Fixes: 7ebb47c9 ("drm/vmwgfx: Read new register for GB memory when available")

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/20240521184720.767-3-ian.forbes@broadcom.com