drm/edid: add a helper for EDID sysfs property show

Add a helper to get the EDID property for sysfs property show. This
hides all the edid_blob_ptr usage within drm_edid.c.

Reviewed-by: Chaitanya Kumar Borah <chaitanya.kumar.borah@intel.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/902c8e09d25b99391fd9c92d95af07c01d7b7cbd.1715353572.git.jani.nikula@intel.com


Signed-off-by: Jani Nikula <jani.nikula@intel.com>