drm/amdkfd: Clamp EOP queue size correctly on Gfx8

Gfx8 HW incorrectly clamps CP_HQD_EOP_CONTROL.EOP_SIZE, which can
lead to scheduling deadlock due to SE EOP done counter overflow.

Enforce a EOP queue size limit which prevents the CP from sending
more than 0xFF events at a time.

Signed-off-by: Jay Cornwall <Jay.Cornwall@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Acked-by: Oded Gabbay <oded.gabbay@gmail.com>
Signed-off-by: Oded Gabbay <oded.gabbay@gmail.com>