drm/i915/edp: fix eDP MSO pipe sanity checks for ADL-P

ADL-P supports stream splitter on pipe B in addition to pipe A. Update
the sanity check in intel_ddi_mso_get_config() to reflect this, and
remove the check in intel_ddi_mso_configure() as redundant with
encoder->pipe_mask. Abstract the splitter pipe mask to a single point of
truth while at it to avoid similar mistakes in the future.

Fixes: 7bc188cc

 ("drm/i915/adl_p: enable MSO on pipe B")
Cc: Uma Shankar <uma.shankar@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Swati Sharma <swati2.sharma@intel.com>
Reviewed-by: Swati Sharma <swati2.sharma@intel.com>
Tested-by: Swati Sharma <swati2.sharma@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210812132354.10885-1-jani.nikula@intel.com