drm/edid: add more general struct edid constness in the interfaces

With this, the remaining non-const parts are the ones that actually
modify the EDID, for example to fix corrupt EDID.

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/c7156f22494b585c55a00a6732462bde0cc19dbf.1648477901.git.jani.nikula@intel.com