drm/rockchip: psr: Sanitize semantics of allow/inhibit API

drm/rockchip: psr: Sanitize semantics of allow/inhibit API

Currently both rockchip_drm_psr_activate() and _deactivate() only set the
boolean "active" flag without actually making sure that hardware state
complies with it.

Since we are going to extend the usage of this API to properly lock PSR
for the duration of atomic commits, we change the semantics in following
way:
 - a counter is used to track the number of inhibit requests,
 - PSR is actually disabled in hardware on first inhibit request,
 - PSR enable work is scheduled on last allow request.

The above allows using the API as a way to deterministically synchronize
PSR state changes with other DRM events, i.e. atomic commits and cursor
updates. As a nice side effect, the naming is sorted out and we have
"inhibit" for stopping the software logic and "enable" for hardware
state.

Signed-off-by: Tomasz Figa <tfiga@chromium.org>
Signed-off-by: Thierry Escande <thierry.escande@collabora.com>
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
Reviewed-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180423105003.9004-26-enric.balletbo@collabora.com