drm/amdkfd: Fix race in gfx10 context restore handler

Missing synchronization with VGPR restore leads to intermittent
VGPR trashing in the user shader.

Signed-off-by: Jay Cornwall <jay.cornwall@amd.com>
Reviewed-by: Yong Zhao <Yong.Zhao@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>