drm/amd/powerplay: add interface to init power (v2)

This patch adds interface to init power function for smu.
It's to create smu_power_context structure, and allocate smu_dpm_context and
context size to fill the smu_power_context data.

v2: add detailed info to describe this function

Signed-off-by: Huang Rui <ray.huang@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>