drm/amd: Refactor find_system_memory()

find_system_memory() pulls out two fields from an SMBIOS type 17
device and sets them on KFD devices. The data offsets are counted
to find interesting data.

Instead use a struct representation to access the members and pull
out the two specific fields.

No intended functional changes.

Link: https://www.dmtf.org/sites/default/files/standards/documents/DSP0134_3.8.0.pdf

 p99
Reviewed-by: Felix Kuehling <felix.kuehling@amd.com>
Link: https://lore.kernel.org/r/20250206214847.3334595-1-superm1@kernel.org


Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>