drm/amd/pm: remove dead code in si_convert_power_level_to_smc

Since gmc_pg is false, setting mcFlags with SISLANDS_SMC_MC_PG_EN  cannot be reach.

Signed-off-by: Jesse Zhang <Jesse.Zhang@amd.com>
Suggested-by: Tim Huang <Tim.Huang@amd.com>
Reviewed-by: Tim Huang <Tim.Huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>