This patch introduces gpu screens into screenInfo. It adds interfaces
for adding and removing gpu screens, along with adding private fixup,
block handler support, and scratch pixmap init.

GPU screens have a myNum that is offset by GPU_SCREEN_OFFSET (256),
this is used for logging etc.

RemoveGPUScreen isn't used until "xfree86: add platform bus hotplug support".

v2: no glyph pictures for GPU screens for now.
v3: introduce MAXGPUSCREENS, fix return value check
v4: fixup myNum when renumbering screens (ajax)
v5: drop cursor privates for now.

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