Add a function to lock memory into RAM and use it in the
gem_tiled_swapping test to reduce the amount of allocated memory
required to force swapping. This also reduces the amount of time
required for the test to complete, since the data set is smaller.

The following durations were recorded with gem_tiled_swapping on a
haswell system before the change:

  Subtest non-threaded: SUCCESS (55.889s)
  Subtest threaded: SUCCESS (810.532s)

and after:

  Subtest non-threaded: SUCCESS (11.804s)
  Subtest threaded: SUCCESS (268.336s)

v2: add various assertions and requirements and make sure
    gem_tiled_swapping works on systems with less RAM (Daniel Vetter)

v3: fix allocation size calculation

Signed-off-by: Thomas Wood <thomas.wood@intel.com>