tests/gem_close_race: Adapt the test for Full PPGTT
tests/gem_close_race: Adapt the test for Full PPGTT

With Full PPGTT, each new fd creates a new context and thus a new
PPGTT, so we have to reduce the number of simultaneous fds or face
OOM problems. For every new PPGTT, its PDEs are stored in the GGTT
which imposes a limit of 1024 new contexts. We want to leave at
least 1/4 of the GGTT available for "important" stuff like scanout
buffers, so never open more than 768 fds.

v2: Unconditionally use the lower limit, as suggested by Daniel
Vetter

Signed-off-by: Oscar Mateo <8d158a6e9567f59e489595fde938b03d223fe815@intel.com>
Signed-off-by: Daniel Vetter <c1b6782c4af8f0673da8923a0702a1832e5940f4@ffwll.ch>

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