drm/amd/powerplay: Use memset to initialize metrics structs

clang warns:

drivers/gpu/drm/amd/amdgpu/../powerplay/navi10_ppt.c:601:33: warning:
suggest braces around initialization of subobject [-Wmissing-braces]
        static SmuMetrics_t metrics = {0};
                                       ^
                                       {}
drivers/gpu/drm/amd/amdgpu/../powerplay/navi10_ppt.c:905:26: warning:
suggest braces around initialization of subobject [-Wmissing-braces]
        SmuMetrics_t metrics = {0};
                                ^
                                {}
2 warnings generated.

One way to fix these warnings is to add additional braces like clang
suggests; however, there has been a bit of push back from some
maintainers[1][2], who just prefer memset as it is unambiguous, doesn't
depend on a particular compiler version[3], and properly initializes all
subobjects. Do that here so there are no more warnings.

[1]: https://lore.kernel.org/lkml/022e41c0-8465-dc7a-a45c-64187ecd9684@amd.com/
[2]: https://lore.kernel.org/lkml/20181128.215241.702406654469517539.davem@davemloft.net/
[3]: https://lore.kernel.org/lkml/20181116150432.2408a075@redhat.com/

Fixes: 98e1a543 ("drm/amd/powerplay: add function get current clock freq interface for navi10")
Fixes: ab43c4bf

 ("drm/amd/powerplay: fix fan speed show error (for hwmon pwm)")
Link: https://github.com/ClangBuiltLinux/linux/issues/583
Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>