drm/amd/powerplay/smu11: disable PLL shutdown when gfxoff enabled

MP1 cannot access clock IP during MP1 FW reload, disable PLL
shutdown as a workaround.

Signed-off-by: Jack Xiao <Jack.Xiao@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>