i965/gen6/xfb: handle case where transform feedback is not active

When the SVBI Payload Enable is false I guess the register R1.4
which contains the Maximum Streamed Vertex Buffer Index is filled by zero
and GS stops to write transform feedback when the transform feedback
is not active.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=107579


Signed-off-by: Andrii Simiklit <andrii.simiklit@globallogic.com>
Reviewed-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>
(cherry picked from commit 1b0df8a4)