drm/i915/display: Add write permissions for fec support

Though there is a write option available on  fec_suport
debugfs file, so far it has been registering with read
permissions only.

Suggested-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Vandita Kulkarni <vandita.kulkarni@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210720064907.9771-2-vandita.kulkarni@intel.com