drm/amdgpu: fix AMDGPU_INFO_READ_MMR_REG

In the Linux kernel, the following vulnerability has been resolved:

drm/amdgpu: fix AMDGPU_INFO_READ_MMR_REG

There were multiple issues in that code.

First of all the order between the reset semaphore and the mm_lock was

wrong (e.g. copy_to_user) was called while holding the lock.

Then we allocated memory while holding the reset semaphore which is also

a pretty big bug and can deadlock.

Then we used down_read_trylock() instead of waiting for the reset to

finish.

(cherry picked from commit 361b6e6b303d4b691f6c5974d3eaab67ca6dd90e)