drm/edid: convert extension block read to EDID block read helper

Use the EDID block read helper also for extension block reads, making
edid_block_read() the only place with the read retry logic.

Note: We observe that drm_do_get_edid() does not use invalid extension
blocks to flag the EDID as corrupt.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/a6328b898db40235b85ad4635374bc0768b5a970.1649685475.git.jani.nikula@intel.com