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

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

drivers/gpu/drm/amd/powerplay/hwmgr/vega10_hwmgr.c: In function vega10_get_performance_level:
drivers/gpu/drm/amd/powerplay/hwmgr/vega10_hwmgr.c:5217:23: warning: variable data set but not used [-Wunused-but-set-variable]

'data' is introduced by commit f688b614 ("drm/amd/pp:
Implement get_performance_level for legacy dgpu"), but never used,
so remove it.

Reviewed-by: Evan Quan <evan.quan@amd.com>
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: zhengbin <zhengbin13@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>