drm/i915/power-well: use generic poll_timeout_us() instead of wait_for() for VLV/CHV

drm/i915/power-well: use generic poll_timeout_us() instead of wait_for() for VLV/CHV

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 500 us sleep instead. The timeout remains at
100 ms.

Reviewed-by: Jouni Högander <jouni.hogander@intel.com>
Link: https://lore.kernel.org/r/c644b7b5611a3c047ea5d3d52acd91830b2fa6b4.1756383233.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>