st/mesa: fix front buffer regression after dropping st_validate_state in Blit
st/mesa: fix front buffer regression after dropping st_validate_state in Blit

Broken by: d082c5324914212f76e45be497229c7a0681f706
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=92072

Cc: 10.6 11.0 <59f39c0db42d4479a46b02d4d2bc11120e37bb44@lists.freedesktop.org>
Tested-by: Ilia Mirkin <bcbbb7aa705ec3a5fc5824c01a845186c3d62fca@alum.mit.edu>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler