drm: trylock modest locking for fbdev panics

In the fbdev code we want to do trylocks only to avoid deadlocks and
other ugly issues. Thus far we've only grabbed the overall modeset
lock, but that already failed to exclude a pile of potential
concurrent operations. With proper atomic support this will be worse.

So add a trylock mode to the modeset locking code which attempts all
locks only with trylocks, if possible. We need to track this in the
locking functions themselves and can't restrict this to drivers since
driver-private w/w mutexes must be treated the same way.

There's still the issue that other driver private locks aren't handled
here at all, but well can't have everything. With this we will at
least not regress, even once atomic allows lots of concurrent kms
activity.

Aside: We should move the acquire context to stack-based allocation in
the callers to get rid of that awful WARN_ON(kmalloc_failed) control
flow which just blows up when memory is short. But that's material for
separate patches.

v2:
- Fix logic inversion fumble in the fb helper.
- Add proper kerneldoc.

Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>