svga: check return value from svga_set_shader( SVGA3D_SHADERTYPE_GS, NULL)

If the call fails we need to flush the command buffer and retry.  In this
case, we were failing to unbind the GS which led to subsequent errors.

This fixes a bug replaying a Cinebench R15 apitrace in a Linux guest.
VMware bug 1894451

cc: mesa-stable@lists.freedesktop.org
Reviewed-by: Charmaine Lee <charmainel@vmware.com>
1 file changed