drm/amdgpu: don't drop the runtime PM reference twice in amdgpu_gpr

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

drm/amdgpu: don't drop the runtime PM reference twice in amdgpu_gpr

amdgpu_debugfs_gpr_read() drops its runtime PM reference right after
reading the wave registers. When copying them to userspace fails, it
jumps to the err label, which drops the reference a second time. A read
into a bad buffer on Navi31 either steals a reference somebody else
holds (power/runtime_usage goes from 1 to 0) or underflows the count:

amdgpu 0000:03:00.0: Runtime PM usage count underflow!

The registers are already in the kernel buffer at that point, so finish
the hardware access first and copy the buffer with a single
copy_to_user() instead of the put_user() loop with its own error path.

(cherry picked from commit 31035bbd74f7c3e10c163db875b8e67f8b50ee6a)