drm/vmwgfx: Calculate the cursor position based on the crtc gui origin

Base the cursor position on the coordinate of the crtc origin in the
gui coordinate system rather than in the framebuffer coordinate system.

With explicit placement, these may differ (for example when two crtcs
scan out of the same framebuffer location).

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