radeonsi: fix a u_blitter crash after a shader with FBFETCH

This fixes an assertion failure with GL CTS when cts-runner is used.
(not a specific test)

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=108877
Cc: 18.3 <mesa-stable@lists.freedesktop.org>
(cherry picked from commit caa2dcd7)