drm/radeon: implement radeon_atif_verify_interface

Wrap the call to VERIFY_INTERFACE and add the parsing of the support
vectors.
v2: use a packed struct for handling the output of ACPI calls, hides
ugly pointer arithmetics (Lee, Chun-Yi <jlee@suse.com>).
v3: fix radeon_atif_parse_functions handling (Alex Deucher)

Signed-off-by: Luca Tettamanti <kronos.it@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>