drm/i915/edp: reject modes with dimensions other than fixed mode

Be more strict about filtering modes for eDP.

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/feb4c3b2b9c4da56a840bdb3c0e7fd0e58ee50de.1613054234.git.jani.nikula@intel.com