drm/amd/powerplay: Remove extra pair of parentheses

The double parentheses are not needed. Removing them fixes the following
warning when building with clang:

drivers/gpu/drm/amd/powerplay/smumgr/tonga_smumgr.c:419:29:
  error: equality comparison with extraneous parentheses
    [-Werror,-Wparentheses-equality]
  if ((data->vdd_gfx_control == SMU7_VOLTAGE_CONTROL_BY_SVID2)) {

Reviewed-by: Guenter Roeck <groeck@chromium.org>
Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>