drm/edid: add function for checking drm_edid validity

We've lacked a function for immutable validity check on drm_edid. Add
one.

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/f96188f64e9f7f3deff348d08296609353b12316.1666614699.git.jani.nikula@intel.com