st/mesa: make KHR_debug output independent of context creation flags (v2)

st/mesa: make KHR_debug output independent of context creation flags (v2)

Instead, keep track of GL_DEBUG_OUTPUT and (un)install the pipe_debug_callback
accordingly. Hardware drivers can still use the absence of the callback to
skip more expensive operations in the normal case, and users can no longer be
surprised by the need to set the debug flag at context creation time.

v2:
- re-add the proper initialization of debug contexts (Ilia Mirkin)
- silence a potential warning (Ilia Mirkin)

Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>