drm/i915/edp: always add fixed mode to probed modes in ->get_modes()

Unconditionally add fixed mode to probed modes even if EDID is present
and has modes. Prepare for cases where the fixed mode is not present in
EDID (such as eDP MSO).

Cc: Nischal Varide <nischal.varide@intel.com>
Reviewed-by: Uma Shankar <uma.shankar@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/6979f123f3e4ed948333f1b181202bbced3c3e85.1613054234.git.jani.nikula@intel.com