mesa: throw error if bufSize negative in GetSynciv on OpenGL ES

mesa: throw error if bufSize negative in GetSynciv on OpenGL ES

Fixes following dEQP tests:

   dEQP-GLES31.functional.debug.negative_coverage.callbacks.state.get_synciv
   dEQP-GLES31.functional.debug.negative_coverage.get_error.state.get_synciv
   dEQP-GLES31.functional.debug.negative_coverage.log.state.get_synciv

v2: drop _mesa_is_gles check (Kenneth)

Signed-off-by: Tapani Pälli <tapani.palli@intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=98133
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>