mod/scmi_clock: Eliminate redundant invocation of get_agent_id

In the current implementation, in all scmi command handler functions,
get_agent_id() of `scmi_api` is being invoked multiple times
redundantly, from another function and explicitly.

To improve efficiency, calling `get_agent_id()` is removed from
`scmi_clock_get_clock_device_entry()` and called explicitly.

Signed-off-by: Md Golam Tanvir Zim <mdgolamtanvir.zim@arm.com>
Change-Id: Ic44adfd6435cd426651cfad3823f2a34bffe59da