drm/probe-helper: abstract .get_modes() connector helper call

Abstract the .get_modes() connector helper call, including the
override/firmware EDID fallback, for clarity.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/a4de51efc246e4f5bcbf7b84d66bb49aaf7fd974.1654674560.git.jani.nikula@intel.com