drm/amd/display: Don't share clk source between DP and HDMI

[why]
Prevent clock source sharing between HDMI and DP connectors.
DP shouldn't be sharing its ref clock with phy clock,
which caused an issue of older ASICS booting up with multiple
diplays plugged in.

[how]
Add an extra check that would prevent HDMI and DP sharing clk.

Signed-off-by: Mikita Lipski <mikita.lipski@amd.com>
Reviewed-by: Hersen Wu <hersenxs.wu@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org