drm/amdgpu: fix documentation around busy_percentage

Add rename the gpu busy percentage for consistency and
add the mem busy percentage documentation.

Reviewed-by: Evan Quan <evan.quan@amd.com>
Reviewed-by: Nirmoy Das <nirmoy.das@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

Documentation/gpu/amdgpu.rst

@@ -197,11 +197,14 @@ pp_power_profile_mode
 .. kernel-doc:: drivers/gpu/drm/amd/amdgpu/amdgpu_pm.c
    :doc: pp_power_profile_mode
 
-busy_percent
-~~~~~~~~~~~~
+*_busy_percent
+~~~~~~~~~~~~~~
 
 .. kernel-doc:: drivers/gpu/drm/amd/amdgpu/amdgpu_pm.c
-   :doc: busy_percent
+   :doc: gpu_busy_percent
+
+.. kernel-doc:: drivers/gpu/drm/amd/amdgpu/amdgpu_pm.c
+   :doc: mem_busy_percent
 
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