ARM: dts: msm: Limit test device context bank use for kona

Ensure that test devices won't try to allocate a context bank
at probe time. Permanently disable the kgsl_coherent_test_device
since there are no spare context banks for it to use.

Change-Id: Ib7a3d7914407dce31d3f0e59e29d9dcd4b9679a1
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>