drm/amd/pm: add a new sysfs entry for default power limit

Driver doesn't keep the default bootup power limit and expose it
to user. As requested we add it in driver.

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