st/dri: no need to request fake front buffer, only handle it being returned

st/dri: no need to request fake front buffer, only handle it being returned

The previous behaviour was incorrect, and resulted in EXT_tfp being broken
for DDX drivers that implement the correct behaviour (intel/radeon/nouveau).

In the cases where a fake front buffer is required, the X server will
return one when requesting __DRI_BUFFER_FRONT_LEFT.

The Xorg state tracker (aka modesetting_drv) is likely broken now until
it's modified to match the other drivers.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>