drm/edid: add EDID block count and size helpers

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