Add a test to verify that pwriting to a future scanout buffer works
correctly. The specific problem occurs when the buffer is already
UC/WT before the pwrite, not the current scanout buffer, and not
currently in the CPU write domain. With the buggy kernel no clflush
will be performed after the pwrite, and hence we end up with cache
dirt on the display.

The problem only affects LLC platforms (non-LLC would clflush anyway
after pwrite), but we can let the test run on all platforms.

v2: Fix typos in commit message and add to .gitignore

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>