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>

drivers/gpu/drm/amd/amdgpu/amdgpu_pm.c

@@ -1668,7 +1668,7 @@ static ssize_t amdgpu_set_pp_power_profile_mode(struct device *dev,
 }
 
 /**
- * DOC: busy_percent
+ * DOC: gpu_busy_percent
  *
  * The amdgpu driver provides a sysfs API for reading how busy the GPU
  * is as a percentage.  The file gpu_busy_percent is used for this.