drm/i915: Move WaPruneModeWithIncorrectHsyncOffset into intel_mode_valid()

Check for the zero length front porch already in intel_mode_valid()
so that we get the same validation for both get_modes() and setcrtc()/etc.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210930104133.30854-3-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>