drm/amdkfd: Fix race in gfx10 context restore handler

drm/amdkfd: Fix race in gfx10 context restore handler

BugLink: https://bugs.launchpad.net/bugs/1900624
[ Upstream commit c18cc2bb9e064d3a613d8276f2cab3984926a779 ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Ian May <ian.may@canonical.com>