drm/amdkfd: Store drm node minor number for kfd nodes

From KFD topology, application will find kfd node with the corresponding
drm device node minor number, for example if partition drm node starts
from /dev/dri/renderD129, then KFD node 0 with store drm node minor
number 129. Application will open drm node /dev/dri/renderD129 to create
amdgpu vm for kfd node 0 with the correct vm->mem_id to indicate the
memory partition.

Signed-off-by: Philip Yang <Philip.Yang@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>