drm/vmwgfx: Implement the cursor_set2 callback v2

Fixes native drm clients like Fedora 23 Wayland which now appears to
be able to use cursor hotspots without strange cursor offsets.
Also fixes a couple of ignored error paths.

Since the core drm cursor hotspot is incompatible with the legacy vmwgfx
hotspot (the core drm hotspot is reset when the drm_mode_cursor ioctl
is used), we need to keep track of both and add them when the device
hotspot is set. We assume that either is always zero.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Sinclair Yeh <syeh@vmware.com>