gallium/winsys/kms: Add support for multi-planes

gallium/winsys/kms: Add support for multi-planes

Add a new struct kms_sw_plane which delegate a plane and use it
in place of sw_displaytarget. Multiple planes share same underlying
kms_sw_displaytarget.

v2:
 - add more check for plane size (Tomasz)
v3:
 - split from larger patch (Emil)
v4:
 - no change from v3
v5:
 - remove mapped field (Tomasz)
v6:
 - remove change-id in commit message (Tomasz)
v7:
 - add revision history in commit message (Emil)

Reviewed-by: Tomasz Figa <tfiga@chromium.org>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Signed-off-by: Lepton Wu <lepton@chromium.org>