media: cec: core: add note about *_from_edid() function usage in drm

In the drm subsystem, the source physical address is, in most cases,
available without having to parse the EDID again. Add notes about
preferring to use the pre-parsed address instead.

Cc: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Cc: linux-media@vger.kernel.org
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Acked-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Link: https://patchwork.freedesktop.org/patch/msgid/20230831105144.25923-1-jani.nikula@intel.com