drm/amdkfd: Allocate doorbells only when needed

Only allocate doorbells when the first queue is created on a GPU or the
doorbells need to be mapped into CPU or GPU virtual address space. This
avoids allocating doorbells unnecessarily and can allow more processes
to use KFD on multi-GPU systems.

Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Reviewed-by: Kent Russell <kent.Russell@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>