drm/edid: constify struct edid passed to detailed blocks

Constify the first level of struct edid in detailed timing parsing. Also
switch to struct edid instead of u8.

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/ec7acb8a9ee0e2868bbb2afdfdd7db114ae180e1.1648477901.git.jani.nikula@intel.com