OMAPDSS: Use PM notifiers for system suspend

The current way how omapdss handles system suspend and resume is that
omapdss device (a platform device, which is not part of the device
hierarchy of the DSS HW devices, like DISPC and DSI, or panels.) uses
the suspend and resume callbacks from platform_driver to handle system
suspend. It does this by disabling all enabled panels on suspend, and
resuming the previously disabled panels on resume.

This presents a few problems.

One is that as omapdss device is not related to the panel devices or the
DSS HW devices, there's no ordering in the suspend process. This means
that suspend could be first ran for DSS HW devices and panels, and only
then for omapdss device. Currently this is not a problem, as DSS HW
devices and panels do not handle suspend.

Another, more pressing problem, is that when suspending or resuming, the
runtime PM functions return -EACCES as runtime PM is disabled during
system suspend. This causes the driver to print warnings, and operations
to fail as they think that they failed to bring up the HW.

This patch changes the omapdss suspend handling to use PM notifiers,
which are called before suspend and after resume. This way we have a
normally functioning system when we are suspending and resuming the
panels.

This patch, I believe, creates a problem that somebody could enable or
disable a panel between PM_SUSPEND_PREPARE and the system suspend, and
similarly the other way around in resume. I choose to ignore the problem
for now, as it sounds rather unlikely, and if it happens, it's not
fatal.

In the long run the system suspend handling of omapdss and panels should
be thought out properly. The current approach feels rather hacky.
Perhaps the panel drivers should handle system suspend, or the users of
omapdss (omapfb, omapdrm) should handle system suspend.

Note that after this patch we could probably revert
0eaf9f52 (OMAPDSS: use sync versions of
pm_runtime_put). But as I said, this patch may be temporary, so let's
leave the sync version still in place.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Reported-by: Jassi Brar <jaswinder.singh@linaro.org>
Tested-by: Jassi Brar <jaswinder.singh@linaro.org>
Tested-by: Joe Woodward <jw@terrafix.co.uk>
Signed-off-by: Archit Taneja <archit@ti.com>
[fts: fixed 2 brace coding style issues]
Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>