dri: Make classic drivers allow __DRI_CTX_FLAG_NO_ERROR.

dri: Make classic drivers allow __DRI_CTX_FLAG_NO_ERROR.

Grigori recently added EGL_KHR_create_context_no_error support,
which causes EGL to pass a new __DRI_CTX_FLAG_NO_ERROR flag to
drivers when requesting an appropriate context mode.

driContextSetFlags() will already handle it properly for us, but the
classic drivers all have code to explicitly balk at unknown flags.  We
need to let it through or they'll fail to create a no_error context.

Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Grigori Goronzy <greg@chown.ath.cx>