drm/i915: Simplify intel_dsc_source_support()

We can simplify the icl check in intel_dsc_source_support()
by noting that the only case when DSC is not supported is when
using transcoder A.

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220124192638.26262-2-ville.syrjala@linux.intel.com