drm/amd/pp: Add powergate_gfx backend function on Raven

Raven support gfx off feature instand of gfx powergate,
so use smu10_gfx_off_control as the powergate_gfx backend function.

Reviewed-by: Evan Quan <evan.quan@amd.com>
Signed-off-by: Rex Zhu <Rex.Zhu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>