A single provider can be both a offload and source slave at the same time,
the use of seperate lists breaks in this case e.g. :

xrandr --listproviders
Providers: number : 2
Provider 0: id: 0x7b cap: 0xf, Source Output, Sink Output, Source Offload, Sink Offload crtcs: 3 outputs: 2 associated providers: 0 name:modesetting
Provider 1: id: 0x46 cap: 0xf, Source Output, Sink Output, Source Offload, Sink Offload crtcs: 2 outputs: 5 associated providers: 0 name:modesetting

xrandr --setprovideroutputsource 1 0x7b
xrandr --listproviders
Providers: number : 2
Provider 0: id: 0x7b cap: 0xf, Source Output, Sink Output, Source Offload, Sink Offload crtcs: 3 outputs: 2 associated providers: 1 name:modesetting
Provider 1: id: 0x46 cap: 0xf, Source Output, Sink Output, Source Offload, Sink Offload crtcs: 2 outputs: 5 associated providers: 1 name:modesetting

xrandr --setprovideroffloadsink 1 0x7b
xrandr --listproviders
Providers: number : 3
Provider 0: id: 0x7b cap: 0xf, Source Output, Sink Output, Source Offload, Sink Offload crtcs: 3 outputs: 2 associated providers: 2 name:modesetting
Provider 1: id: 0x46 cap: 0xf, Source Output, Sink Output, Source Offload, Sink Offload crtcs: 2 outputs: 5 associated providers: 2 name:modesetting
Provider 2: id: 0x46 cap: 0xf, Source Output, Sink Output, Source Offload, Sink Offload crtcs: 2 outputs: 5 associated providers: 2 name:modesetting

Not good. The problem is that the provider with id 0x46 now is on both
the output_slave_list and the offload_slave_list of the master screen.

This commit fixes this by unifying all 3 lists into a single slaves list.

Note that this does change the struct _Screen definition, so this is an ABI
break. I do not expect any of the drivers to actually use the removed / changed
fields so a recompile should suffice.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>