drm/bridge: ps8640: Use constant sleep time for polling hpd

The default hpd_wait_us in panel_edp.c is 2 seconds. This makes the
sleep time in the polling of _ps8640_wait_hpd_asserted become 200ms.
Change it to a constant 20ms to speed up the function.

Signed-off-by: Pin-yen Lin <treapking@chromium.org>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Robert Foss <rfoss@kernel.org>
Signed-off-by: Robert Foss <rfoss@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230331030204.1179524-1-treapking@chromium.org