drm/amdgpu: add info ioctl query for vce clock info (v3)

This is needed to set up the vce clock table in userspace
for proper VCE DPM.

v2: fix copy paste typo in comment
v3: track number of valid states

Reviewed-by: Rex Zhu <Rex.Zhu@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>