drm: add drm_edid_to_eld helper extracting SADs from EDID (v2)

Some devices (ATI/AMD cards) don't support passing ELD struct to the
hardware but just require filling specific registers and then the
hardware/firmware does the rest. In such cases we need to read the info
from SAD blocks and put them in the correct registers.

agd5f: note that the returned pointer needs to be kfreed as per
Christian's suggestion.

v2: fix warning

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>