drm/msm: fix potential deadlock in gpu init

Somewhere along the way, the firmware loader sprouted another lock
dependency, resulting in possible deadlock scenario:

 &dev->struct_mutex --> &sb->s_type->i_mutex_key#2 --> &mm->mmap_sem

which is problematic vs things like gem mmap.

So introduce a separate mutex to synchronize gpu init.

Signed-off-by: Rob Clark <robdclark@gmail.com>