drm/vmwgfx: Don't overwrite KMS surface dirty tracker

drm/vmwgfx: Don't overwrite KMS surface dirty tracker

We were overwriting the surface's dirty tracker here causing a memory leak.

Reported-by: Mika Penttilä <mpenttil@redhat.com>
Closes: https://lore.kernel.org/dri-devel/8c53f3c6-c6de-46fe-a8ca-d98dd52b3abe@redhat.com/
Fixes: 965544150d1c ("drm/vmwgfx: Refactor cursor handling")
Signed-off-by: Ian Forbes <ian.forbes@broadcom.com>
Reviewed-by: Maaz Mombasawala <maaz.mombasawala@broadcom.com>
Signed-off-by: Zack Rusin <zack.rusin@broadcom.com>
Link: https://patch.msgid.link/20260302200330.66763-1-ian.forbes@broadcom.com