CVE-2026-99150: 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 amdgpugpr

amdgpudebugfsgprread() 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/runtimeusage 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 copytouser() instead of the putuser() loop with its own error path.

(cherry picked from commit 31035bbd74f7c3e10c163db875b8e67f8b50ee6a)