drm/amd/pm: drop the incomplete fix for Navi14 runpm issue

As the fix by adding PPSMC_MSG_PrepareMp1ForUnload is proved to
be incomplete. Another fix(see link below) has been sent out.
Link: https://lore.kernel.org/linux-pci/20210602021255.939090-1-evan.quan@amd.com/



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