drm/i915/ehl: unconditionally flush the pages on acquire

EHL and JSL add the 'Bypass LLC' MOCS entry, which should make it
possible for userspace to bypass the GTT caching bits set by the kernel,
as per the given object cache_level. This is troublesome since the heavy
flush we apply when first acquiring the pages is skipped if the kernel
thinks the object is coherent with the GPU. As a result it might be
possible to bypass the cache and read the contents of the page directly,
which could be stale data. If it's just a case of userspace shooting
themselves in the foot then so be it, but since i915 takes the stance of
always zeroing memory before handing it to userspace, we need to prevent
this.

v2: this time actually set cache_dirty in put_pages()
v3: move to get_pages() which looks simpler

BSpec: 34007
References: 046091758b50 ("Revert "drm/i915/ehl: Update MOCS table for EHL"")
Signed-off-by: Matthew Auld <[email protected]>
Cc: Tejas Upadhyay <[email protected]>
Cc: Francisco Jerez <[email protected]>
Cc: Lucas De Marchi <[email protected]>
Cc: Jon Bloomfield <[email protected]>
Cc: Chris Wilson <[email protected]>
Cc: Matt Roper <[email protected]>
Cc: Daniel Vetter <[email protected]>
Reviewed-by: Daniel Vetter <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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