drm/amdkfd: Allow access for mmapping KFD BOs

DRM render node file handles are used for CPU mapping of BOs using mmap
by the Thunk. It uses the DRM render node of the GPU where the BO was
allocated.

DRM allows mmap access automatically when it creates a GEM handle for a
BO. KFD BOs don't have GEM handles, so KFD needs to manage access
manually. Use drm_vma_node_allow to allow user mode to mmap BOs allocated
with kfd_ioctl_alloc_memory_of_gpu through the DRM render node that was
used in the kfd_ioctl_acquire_vm call for the same GPU.

Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Philip Yang <philip.yang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>