drm/amdgpu: fix AMDGPU_INFO_READ_MMR_REG

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.

Signed-off-by: Christian König <christian.koenig@amd.com>
Fixes: 9e823f307074 ("drm/amdgpu: Block MMR_READ IOCTL in reset")
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>