drm/amdkfd: Fix CP soft hang on APUs

The problem happens on Raven and Carrizo. The context save handler
should not clear the high bits of PC_HI before extracting the bits
of IB_STS.

The bug is not relevant to VEGA10 until we enable demand paging.

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