lib/igt_kms: use struct edid instead of unsigned char

This has several advantages:

* No more need to convert back and forth between these two (everybody should
  use struct edid, the exception being lib/tests/igt_edid which performs sanity
  checks)
* Makes it clearer that users can call edid_get_size on a returned EDID blob
* Improves type safety (it's more obvious is a random blob is used as an EDID)

Signed-off-by: Simon Ser <simon.ser@intel.com>
Reviewed-by: Arkadiusz Hiler <arkadiusz.hiler@intel.com>