drm/i915/lspcon: use generic poll_timeout_us() instead of wait_for()

drm/i915/lspcon: use generic poll_timeout_us() instead of wait_for()

Prefer generic poll helpers over i915 custom helpers.

The functional change is losing the exponentially growing sleep of
wait_for(), which used to be 10, 20, 40, ..., 640, and 1280 us.

Use an arbitrary constant 5 ms sleep instead. The timeouts remain, being
400 ms or 800 ms, depending on the case.

Reviewed-by: Jouni Högander <jouni.hogander@intel.com>
Link: https://lore.kernel.org/r/4065fa96c0ef6afd51a384f365761d2ca802256b.1756383233.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>