drm/amd/pm: revise the umc hybrid cdr workaround

Drop the unused message(SMU_MSG_DAL_DISABLE_DUMMY_PSTATE_CHANGE).
And do not apply this workaround when the max uclk frequency
is greater than 750Mhz.

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>