drm/displayid: add new displayid section/block iterators

Iterating DisplayID blocks across sections (in EDID extensions) is
unnecessarily complicated for the caller. Implement DisplayID iterators
to go through all blocks in all sections.

Usage example:

	const struct displayid_block *block;
	struct displayid_iter iter;

	displayid_iter_edid_begin(edid, &iter);
	displayid_iter_for_each(block, &iter) {
		/* operate on block */
	}
	displayid_iter_end(&iter);

When DisplayID is stored in EDID extensions, the DisplayID sections map
to extensions as described in VESA DisplayID v1.3 Appendix B: DisplayID
as an EDID Extension. This is implemented here.

When DisplayID is stored in its dedicated DDC device 0xA4, according to
VESA E-DDC v1.3, different rules apply for the structure. This is not
implemented here, as we don't currently use it, but the idea is you'd
have a different call for beginning the iteration, for example simply:

	displayid_iter_begin(displayid, &iter);

instead of displayid_iter_edid_begin(), and everything else would be
hidden away in the iterator functions.

v2:
- sizeof(struct displayid_block) -> sizeof(*block) (Ville)
- remove __ prefix from displayid_iter_block

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/da3dead1752ab16c061f7bd248ac1a4268f7fefb.1617024940.git.jani.nikula@intel.com