drm/amdkfd: EOP Removal - Handle size 0 correctly

On GC 9.4.3, we are removing the EOP buffer.
If we specify 0 for the size, CP_HQD_EOP_CONTROL ends up with
incorrect value as order_size_2 calculations does not handle 0.

Fix it by using zero for the MQD entry for EOP size 0.

v2: Reworked code with a conditional assignment and fixed style issues.

Signed-off-by: David Belanger <david.belanger@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>