drm/tyr: use read_poll_timeout

drm/tyr: use read_poll_timeout

The L2 power-on sequence and soft reset in Tyr previously relied on fixed
sleeps followed by a single register check, since polling helpers were not
available in Rust at the time.

Now that read_poll_timeout() is available, poll the relevant registers
until the hardware reports readiness or a timeout is reached.

This avoids unnecessary delays on start-up.

Signed-off-by: Deborah Brouwer <deborah.brouwer@collabora.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Link: https://patch.msgid.link/20260119202645.362457-1-deborah.brouwer@collabora.com
Signed-off-by: Alice Ryhl <aliceryhl@google.com>