drm/amd/powerplay: drop unnecessary SMU_MSG_GetDpmClockFreq check

Since SMU_MSG_GetDpmClockFreq is known to be supported for Vega20
and before ASICs only. For those ASICs supporting swSMU, it is not
supported.

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