omapdrm: simplify locking in the fb debugfs file

We don't need to hold onto mode_config.mutex any more to keep the fb
objects around. And locking dev->struct_mutex is also not required,
since omap_gem_describe only reads data anyway. And for a debug
interface it's better to grab fewer locks in case the driver is
deadlocked already ...

The only thing we need is to hold onto mode_config.fb_lock to ensure
the user-created fbs don't disappear. The fbcon fb doesn't need any
protection, since it lives as long as the driver (and so the debugfs
files) itself. And if the teardown/setup isn't following the right
sequence grabbing locks won't prevent a NULL deref on priv->fbdev if
the fb is not yet (or no longer) there.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Rob Clark <robdclark@gmail.com>