drm/amdkfd: Use VMID bitmap from KGD v2

The hard-coded values related to VMID were removed in KFD, as those
values can be calculated in the KFD initialization function.

v2: remove unnecessary local variable

Signed-off-by: Yong Zhao <yong.zhao@amd.com>
Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Reviewed-by: Oded Gabbay <oded.gabbay@gmail.com>
Signed-off-by: Oded Gabbay <oded.gabbay@gmail.com>