drm/exynos/mixer: fix MIXER shadow registry synchronisation code

[ Upstream commit 6a3b45ada960ac475ec2b4103d43e57943b2b8d3 ]

MIXER on Exynos5 SoCs uses different synchronisation method than Exynos4
to update internal state (shadow registers).
Apparently the driver implements it incorrectly. The rule should be
as follows:
- do not request updating registers until previous request was finished,
  ie. MXR_CFG_LAYER_UPDATE_COUNT must be 0.
- before setting registers synchronisation on VSYNC should be turned off,
  ie. MXR_STATUS_SYNC_ENABLE should be reset,
- after finishing MXR_STATUS_SYNC_ENABLE should be set again.
The patch hopefully implements it correctly.
Below sample kernel log from page fault caused by the bug:

[   25.670038] exynos-sysmmu 14650000.sysmmu: 14450000.mixer: PAGE FAULT occurred at 0x2247b800
[   25.677888] ------------[ cut here ]------------
[   25.682164] kernel BUG at ../drivers/iommu/exynos-iommu.c:450!
[   25.687971] Internal error: Oops - BUG: 0 [#1] PREEMPT SMP ARM
[   25.693778] Modules linked in:
[   25.696816] CPU: 5 PID: 1553 Comm: fb-release_test Not tainted 5.0.0-rc7-01157-g5f86b1566bdd #136
[   25.705646] Hardware name: SAMSUNG EXYNOS (Flattened Device Tree)
[   25.711710] PC is at exynos_sysmmu_irq+0x1c0/0x264
[   25.716470] LR is at lock_is_held_type+0x44/0x64

v2: added missing MXR_CFG_LAYER_UPDATE bit setting in mixer_enable_sync

Reported-by: Marian Mihailescu <mihailescu2m@gmail.com>
Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>