drm/edid: split out drm_eld.h from drm_edid.h

The drm_edid.[ch] files are starting to be a bit crowded, and with plans
to add more ELD related functionality, it's perhaps cleanest to split
the ELD code out to a header of its own.

Include drm_eld.h from drm_edid.h for starters, and leave it to
follow-up work to only include drm_eld.h where needed.

Cc: Mitul Golani <mitulkumar.ajitkumar.golani@intel.com>
Reviewed-by: Mitul Golani <mitulkumar.ajitkumar.golani@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/0c6d631fa1058036d72dd25d1cabc90a7c52490e.1698747331.git.jani.nikula@intel.com