drm/amdkfd: remove unused struct cdit_header

struct cdit_header was never used across any of the amd drivers nor
this is exposed to UAPI so it can be removed.

This patch removes struct cdit_header and refactor code accordingly

Signed-off-by: Paulo Miguel Almeida <paulo.miguel.almeida.rodenas@gmail.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>