drm/amd/pp: Refine code abbreviate variable name

abbreviate variable name number_of_performance_levels
to num_of_pl in struct phm_odn_clock_levels

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Rex Zhu <Rex.Zhu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>