mesa/main: fix validation of ARB_query_buffer_object

mesa/main: fix validation of ARB_query_buffer_object

ctx->Extensions.ARB_query_buffer_object is set based on the driver-
capabilities, not based on the context type. We need to check against
_mesa_has_ARB_query_buffer_object(ctx) instead to figure out if the
extension is really supported.

This turns attempts to read queries into buffer objects on ES 3 into
errors, as required by the spec.

Signed-off-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Reviewed-by: Tapani Pälli <tapani.palli@intel.com>