drm/i915: Remove DRM_MODE_FLAG_DBLSCAN checks from .mode_valid() hooks

We never set connector->doublescan_allowed, so the probe helper
already filters out all doublescan modes for us.

Sadly we still need to keep the explicit doublescan checks
in .compute_config as outlined in commit e4dd27aa


("drm/i915: Allow DBLSCAN user modes with eDP/LVDS/DSI")

Reviewed-by: Vandita Kulkarni <vandita.kulkarni@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240402135148.23011-2-ville.syrjala@linux.intel.com