drivers: gpu: Remove unused function in ttm_lock.c

Remove unused function ttm_write_lock_downgrade() from
drm/ttm/ttm_lock.c.

This eliminates the following warning in drm/ttm/ttm_lock.c:
drivers/gpu/drm/ttm/ttm_lock.c:189:6: warning: no previous prototype for ‘ttm_write_lock_downgrade’ [-Wmissing-prototypes]

Signed-off-by: Rashika Kheria <rashika.kheria@gmail.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Reviewed-by: Thomas Hellstrom <thellstrom@vmware.com>