drivers: mipi_dbi: lower ROM usage by switching reset time to k_timeout_t

Calling k_msleep() leads to a int64_t division, causing udivdi3 and
similar to be linked in.

Keep the outer API the same, but switch to k_timeout_t before passing
to the inner API. This saves 1916 B of ROM.

Signed-off-by: Michael Hope <mlhx@google.com>