drm/amd/pm: Return 0 as default min power limit for legacy asics

Return 0 as the default min power limit for the asics use
powerplay.

Signed-off-by: Ma Jun <Jun.Ma2@amd.com>
Reviewed-by: Kenneth Feng <kenneth.feng@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>