drm/i915/mtl: Fix SSC selection for MPLLA

Driver does not clear the default SSC for MPLLA. This causes link training
failure when trying to use 10G and 20G rates. Fix the behaviour and enable
ssc only when we really want.

Fixes: 237e7be0 ("drm/i915/mtl: For DP2.0 10G and 20G rates use MPLLA")
Cc: Mika Kahola <mika.kahola@intel.com>
Cc: Clint Taylor <Clinton.A.Taylor@intel.com>
Cc: Khaled Almahallawy <khaled.almahallawy@intel.com>
Cc: Arun R Murthy <arun.r.murthy@intel.com>
Signed-off-by: Radhakrishna Sripada <radhakrishna.sripada@intel.com>
Tested-by: Khaled Almahallawy <khaled.almahallawy@intel.com>
Reviewed-by: Mika Kahola <mika.kahola@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230616043950.1576836-1-radhakrishna.sripada@intel.com