mesa: Use ctx->ReadBuffer in glReadBuffer back-to-front tests

mesa: Use ctx->ReadBuffer in glReadBuffer back-to-front tests

We were looking at ctx->DrawBuffer when asking about the read buffer,
which was good enough for CTS purposes, but definitely not right.

Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>
Reviewed-by: Kristian H. Kristensen <hoegsberg@google.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>