drm/vmwgfx: Update last_read_seqno under the fence lock

There was a possible race in vmw_update_seqno. Because of this race it
was possible for last_read_seqno to go backwards. Remove this function
and replace it with vmw_update_fences which now sets and returns the
last_read_seqno while holding the fence lock. This serialization via the
fence lock ensures that last_read_seqno is monotonic again.

Signed-off-by: Ian Forbes <ian.forbes@broadcom.com>
Signed-off-by: Zack Rusin <zack.rusin@broadcom.com>
Link: https://lore.kernel.org/r/20250530183510.733175-1-ian.forbes@broadcom.com