st/mesa: fix glReadBuffer() assertion failure

st/mesa: fix glReadBuffer() assertion failure

If the first call in a GL app is glReadPixels(GL_FRONT) we'd fail the
assert(st->ctx->FragmentProgram._Current) at st_atom_shader.c:114 in
update_fp().

This is because we were calling st_validate_state() without first
updating Mesa state with _mesa_update_state().

The regression came from commit 83b589301f4a150f4 "st/mesa: fix
frontbuffer glReadPixels regressions".

The new piglit gl-1.0-simple-readbuffer test exercises this.

Cc: "11.1 11.2" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Reviewed-by: Jose Fonseca <jfonseca@vmware.com>