drm/amdgpu: Set reference clock to 100Mhz on Renoir (v2)

Fixes the rlc reference clock used for GPU timestamps.
Value is 100Mhz.  Confirmed with hardware team.

v2: reword commit message.

Bug: drm/amd#1480


Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org