dri2: do not conflate unbind and bindContext() failure
dri2_make_current() has become hard to follow, address this by splitting the semantic of needing a call to bindContext() and its failure. Cc: mesa-stable Signed-off-by:Luigi Santivetti <luigi.santivetti@imgtec.com> Reviewed-by:
Eric Engestrom <eric@engestrom.ch> Reviewed-by:
Emil Velikov <emil.velikov@collabora.com> Part-of: <mesa/mesa!5707> (cherry picked from commit 8b0b6f90)