drm/vmwgfx: Fix Host-Backed userspace on Guest-Backed kernel

Running 3D applications with SVGA_FORCE_HOST_BACKED=1 or using an
ancient version of mesa was broken because the buffer was pinned in
VMW_BO_DOMAIN_SYS and could not be moved to VMW_BO_DOMAIN_MOB during
validation.

The compat_shader buffer should not pinned.

Fixes: 668b2066 ("drm/vmwgfx: Stop using raw ttm_buffer_object's")
Signed-off-by: Ian Forbes <ian.forbes@broadcom.com>
Reviewed-by: Maaz Mombasawala <maaz.mombasawala@broadcom.com>
Signed-off-by: Zack Rusin <zack.rusin@broadcom.com>
Link: https://lore.kernel.org/r/20250429203427.1742331-1-ian.forbes@broadcom.com