Add some helpers to figure out the EDID extension block count, block
count, size, pointers to blocks.

Unfortunately, we'll need to cast away the const in a few places where
we actually need to access the data.

v3: fix (!edid_extension_block_count(edid) == 0) (kernel test robot)

v2: fix s/j/i/ introduced in a rebase
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/dc7b0850293d837439fb3914c8a9d81e39018b4b.1649685475.git.jani.nikula@intel.com