tests/i915/gem_softpin: Add safe-alignment test

Exercise start offset and alignment detection when we start mixing
system and local memory.

v2: support integrated and check smem <-> smem alignment
v3: iterate over engines to verify safe start is correct everywhere
v4: rename to safe-alignment (Ashutosh)

Signed-off-by: Zbigniew Kempczyński <zbigniew.kempczynski@intel.com>
Cc: Petri Latvala <petri.latvala@intel.com>
Cc: Ashutosh Dixit <ashutosh.dixit@intel.com>
Reviewed-by: Ashutosh Dixit <ashutosh.dixit@intel.com>