x11-drivers/xorgxrdp: disable DRI3 option by default again

I tried to compile DRI3 support by default and disable by config by default [1]
However, actually it cannot be disabled completely by config. Hardware
acceleration in xrdp is still bleeding-edge and it sometimes causes hang-up
like below.

[drm] GPU HANG: ecode 7:0:0x97f4ffff, in Xorg [100659], reason: Hang on rcs0, action: reset
drmn0: Resetting chip after gpu hang
drmn0: Resetting chip after gpu hang
drmn0: i915_reset_device timed out, cancelling all in-flight rendering.

Bleeding-edge features should be opt-in and the default setup should be stable,
disabling again.

[1] https://gitter.im/neutrinolabs/xrdp?at=5e5e48163ca0375cb399cdfa