drm/bridge: analogix_dp: add the PSR function support

The full name of PSR is Panel Self Refresh, panel device could refresh
itself with the hardware framebuffer in panel, this would make lots of
sense to save the power consumption.

This patch have exported two symbols for platform driver to implement
the PSR function in hardware side:
- analogix_dp_active_psr()
- analogix_dp_inactive_psr()

Reviewed-by: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Yakir Yang <ykk@rock-chips.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>