drm/amdgpu: modify system reference clock source for navi+ (V2)

Starting from Navi+, the rlc reference clock is used for system clock
from vbios gfx_info table. It is incorrect to use core_refclk_10khz of
vbios smu_info table as system clock.

Signed-off-by: Aaron Liu <aaron.liu@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Acked-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>