drm/edid: split out edid_header_fix()

Give a name to the EDID header fixup instead of having an inline memcpy.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
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/c10c24b179e942548cd5d693d159bb0d12be4d26.1648752228.git.jani.nikula@intel.com