tests/gem_mmap_gtt: Make the small-bo tiling tests work on old platforms

Several factors conspire against us when trying to execute
the tiled small-bo tests:
- pre-gen4 require power of two fences, with natural alignment
- the entire gtt may be mappable
- we put a guard page at the end of gtt

What all that means is that when we try to use a tiled object half
the size of the mappable area, we can only fit it in the first half
of the gtt. That leads to a SIGBUS when we try to fault in the
object when there's already something (eg. fbdev) occupying the
first half of gtt.

So in order to make the tests run on old machines, let's further
halve the object size when things look too tight.

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

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