drm/ast: astdp: fix loop timeout check

This code has an issue because it loops until "i" is set to UINT_MAX but
the test for failure assumes that "i" is set to zero.  The result is that
it will only print an error message if we succeed on the very last try.
Reformat the loop to count forwards instead of backwards.

Fixes: 22814751 ("drm/ast: astdp: Perform link training during atomic_enable")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/1ba8da25-2d09-4924-a4ff-c0714bfbb192@stanley.mountain