drm/amdkfd: Preserve ttmp[4:5] instead of ttmp[14:15]

ttmp[4:5] is initialized by the SPI with SPI_GDBG_TRAP_DATA* values.
These values are more useful to the debugger than ttmp[14:15], which
carries dispatch_scratch_base*. There are too few registers to
preserve both.

Signed-off-by: Jay Cornwall <Jay.Cornwall@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>