drm/amd/powerplay: remove set but not used variable 'us_mvdd'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/gpu/drm/amd/powerplay/smumgr/vegam_smumgr.c: In
function ‘vegam_populate_smc_acpi_level’:
drivers/gpu/drm/amd/powerplay/smumgr/vegam_smumgr.c:1117:11:
warning: variable 'us_mvdd' set but not used [-Wunused-but-set-variable]

It is never used, so can be removed.

Fixes: ac7822b0 ("drm/amd/powerplay: add smumgr support for VEGAM (v2)")
Signed-off-by: yu kuai <yukuai3@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>