drm/amdkfd: Allow CPU access for all VRAM BOs

The thunk needs to mmap all BOs for CPU access to allow the debugger to
access them. Invisible ones are mapped with PROT_NONE.

Fixes: 71df0368

 ("drm/amdgpu: Implement mmap as GEM object function")
Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>