mesa: wire up EmitStringMarker for KHR_debug

mesa: wire up EmitStringMarker for KHR_debug

The extension spec[1] describes DEBUG_TYPE_MARKER as "Annotation of the
command stream".  So for DEBUG_TYPE_MARKER, also pass the buf to the
driver's EmitStringMarker() to be inserted in the command stream.

[1] https://www.opengl.org/registry/specs/KHR/debug.txt

Signed-off-by: Rob Clark <robclark@freedesktop.org>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>