drm/amdgpu: refine get gpu clock counter method

[why]
regGOLDEN_TSC_COUNT_LOWER/regGOLDEN_TSC_COUNT_UPPER are protected and
unaccessible under sriov.
The clock counter high bit may update during reading process.

[How]
Replace regGOLDEN_TSC_COUNT_LOWER/regGOLDEN_TSC_COUNT_UPPER with
regCP_MES_MTIME_LO/regCP_MES_MTIME_HI to get gpu clock under sriov.
Refine get gpu clock counter method to make the result more precise.

Signed-off-by: Tong Liu01 <Tong.Liu01@amd.com>
Acked-by: Luben Tuikov <luben.tuikov@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>