drm/mgag200: Add a workaround for low-latency

We found a regression in v5.10 on real-time server, using the
rt-kernel and the mgag200 driver. It's some really specialized
workload, with <10us latency expectation on isolated core.
After the v5.10, the real time tasks missed their <10us latency
when something prints on the screen (fbcon or printk)

The regression has been bisected to 2 commits:
commit 0b34d58b6c32 ("drm/mgag200: Enable caching for SHMEM pages")
commit 4862ffaec523 ("drm/mgag200: Move vmap out of commit tail")

The first one changed the system memory framebuffer from Write-Combine
to the default caching.
Before the second commit, the mgag200 driver used to unmap the
framebuffer after each frame, which implicitly does a cache flush.
Both regressions are fixed by this commit, which restore WC mapping
for the framebuffer in system memory, and add a cache flush.
This is only needed on x86_64, for low-latency workload,
so the new kconfig DRM_MGAG200_IOBURST_WORKAROUND depends on
PREEMPT_RT and X86.

For more context, the whole thread can be found here [1]

Signed-off-by: Jocelyn Falempe <jfalempe@redhat.com>
Link: https://lore.kernel.org/dri-devel/20231019135655.313759-1-jfalempe@redhat.com/ # 1
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20240208095125.377908-1-jfalempe@redhat.com

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