drm/amd/display: Check if clock source in use before disabling

[why]
We are disabling clock source while other pipes are still using
it, because we don't verify the number of pipes that share it.

[how]
- Adding a function in resources to return the number of pipes
sharing the clock source.
- Checking that no one is sharing the clock source before disabling

Signed-off-by: Mikita Lipski <mikita.lipski@amd.com>
Reviewed-by: Harry Wentland <Harry.Wentland@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org