drm/edid: constify struct detailed_timing in parsing callbacks

Moving one level higher, constify struct detailed_timing pointers in
callbacks.

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>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/9b617068d2349a574a837ad6207b1d45c4d79eb5.1648477901.git.jani.nikula@intel.com