drm/edid: track invalid blocks in drm_do_get_edid()

Track invalid blocks instead of valid extensions to minimize impact on
the happy day scenario, and hide the details in the separate function.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
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/6215f85b01c579a44c66129d2b5f41e1ab9294de.1648752228.git.jani.nikula@intel.com