mesa/st: emit string marker based on PIPE_CAP_STRING_MARKER
Fixes: 10ac88b7 ("mesa/st: drop emit string marker device table entry.")
Noticed this when using zink over venus which doesn't expose EXT_debug_utils.
Fixed below test failures against Zink:
- dEQP-GLES31.functional.debug.externally_generated#application_messages
- dEQP-GLES31.functional.debug.externally_generated#third_party_messages
- dEQP-GLES31.functional.debug.error_* 32 failures