drm/amdkfd: Fix kernel-queue wrapping bugs

Avoid intermediate negative numbers when doing calculations with a mix
of signed and unsigned variables where implicit conversions can lead
to unexpected results.

When kernel queue buffer wraps around to 0, we need to check that rptr
won't be overwritten by the new packet.

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