enable vcn powergating in driver for navi10

v2: set vcn pg bit according to AMD_PG_SUPPORT_VCN flag
Signed-off-by: Kenneth Feng <kenneth.feng@amd.com>
Signed-off-by: Jack Xiao <Jack.Xiao@amd.com>
Reviewed-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>