amdgpu: fix Documentation builds for pm/ file movement

Fix Documentation errors for amdgpu.rst due to file rename (moved
to another subdirectory).

Error: Cannot open file ../drivers/gpu/drm/amd/amdgpu/amdgpu_pm.c
WARNING: kernel-doc '../scripts/kernel-doc -rst -enable-lineno -function hwmon ../drivers/gpu/drm/amd/amdgpu/amdgpu_pm.c' failed with return code 1

Fixes: e098bc96 ("drm/amd/pm: optimize the power related source code layout")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Evan Quan <evan.quan@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Evan Quan <evan.quan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>