drm/amd/powerplay: initialize a variable before using it

Function vega10_apply_state_adjust_rules() only initializes
stable_pstate_sclk_dpm_percentage when
data->registry_data.stable_pstate_sclk_dpm_percentage is not between 1
and 100. The variable is then used to compute stable_pstate_sclk, which
therefore uses an uninitialized value.

Fix this by initializing stable_pstate_sclk_dpm_percentage to
data->registry_data.stable_pstate_sclk_dpm_percentage.

This issue has been found while building the kernel with clang. The
compiler reported a -Wsometimes-uninitialized warning.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Fixes: f83a9991 ("drm/amd/powerplay: add Vega10 powerplay support (v5)")
Signed-off-by: Nicolas Iooss <nicolas.iooss_linux@m4x.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>