drm/i915/mtl: Fix HDMI/DP PLL clock selection

Select the HDMI specific PLL clock only for HDMI outputs.

Fixes: 62618c7f ("drm/i915/mtl: C20 PLL programming")
Cc: Mika Kahola <mika.kahola@intel.com>
Cc: Radhakrishna Sripada <radhakrishna.sripada@intel.com>
Reviewed-by: Radhakrishna Sripada <radhakrishna.sripada@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231213220526.1828827-1-imre.deak@intel.com