drm/i915/dp: Some reshuffling in mode_valid as prep for bigjoiner modes

No functional changes. This patch just moves some mode checks
around to prepare for adding bigjoiner related mode validation

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Manasi Navare <manasi.d.navare@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201112023954.12301-1-manasi.d.navare@intel.com