intel_fb_drv: make buffering mandatory

Instead of providing a buffer to the client and blitting from that
to the "real" framebuffer as an option, with this commit we always do so.
Thereby its possible to immediately destroy the old framebuffer used by
hardware when a configuration change is done, and a new framebuffer
is used. This also simplifies the modesetting.
Moreover, this commit fixes an issue when not using the connector reporting.
Until now the initial mode detection of connectors was only done when
the report was created. this is a regression that entered the driver
when upgrading to the recent Linux kernel version.

Ref #1997

repos/dde_linux/src/test/framebuffer/intel/main.cc

@@ -84,7 +84,6 @@ void Framebuffer_controller::update_fb_config(Xml_node & report)
 		static char buf[4096];
 
 		Xml_generator xml(buf, sizeof(buf), "config", [&] {
-			xml.attribute("buffered", "yes");
 			xml.node("report", [&] {
 				xml.attribute("connectors", "yes");
 			});