drm: Restore double clflush on the last partial cacheline

drm: Restore double clflush on the last partial cacheline

This effectively reverts

commit afcd950cafea6e27b739fe7772cbbeed37d05b8b
Author: Chris Wilson <[email protected]>
Date:   Wed Jun 10 15:58:01 2015 +0100

    drm: Avoid the double clflush on the last cache line in drm_clflush_virt_range()

as we have observed issues with serialisation of the clflush operations
on Baytrail+ Atoms with partial updates. Applying the double flush on the
last cacheline forces that clflush to be ordered with respect to the
previous clflush, and the mfence then protects against prefetches crossing
the clflush boundary.

The same issue can be demonstrated in userspace with igt/gem_exec_flush.

Fixes: afcd950cafea6 (drm: Avoid the double clflush on the last cache...)
Testcase: igt/gem_concurrent_blit
Testcase: igt/gem_partial_pread_pwrite
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=92845
Signed-off-by: Chris Wilson <[email protected]>
Cc: [email protected]
Cc: Akash Goel <[email protected]>
Cc: Imre Deak <[email protected]>
Cc: Daniel Vetter <[email protected]>
Cc: Jason Ekstrand <[email protected]>
Cc: [email protected]
Reviewed-by: Mika Kuoppala <[email protected]>
Signed-off-by: Daniel Vetter <[email protected]>
Link: http://patchwork.freedesktop.org/patch/msgid/[email protected]