Revert "dix: Remove usage_hint from pixmaps, store it in ->drawable.class"

This reverts commit 1564c82417.

The drivers used the top bits of the usage_hint to store driver
private flags (intel, radeon, nouveau).

With EXA we need to get at this data so if we migrate the pixmap we
can create the correct type of pixmap in the driver, however this
commit truncates the usage_hint into 8-bit class and loses all the
good stuff.

Signed-off-by: Dave Airlie <airlied@gmail.com>
Reviewed-by: Keith Packard <keithp@keithp.com>

include/pixmapstr.h

@@ -80,6 +80,7 @@ typedef struct _Pixmap {
     short		screen_x;
     short		screen_y;
 #endif
+    unsigned		usage_hint; /* see CREATE_PIXMAP_USAGE_* */
 } PixmapRec;
 
 #endif /* PIXMAPSTRUCT_H */