i915/gem_create: use atomic_* instead of __sync_*

This fixes builds on some architectures, in particular MIPS which
doesn't have __sync_add_and_fetch_8 and __sync_val_compare_and_swap_8
for 64-bit variable handling.

* replace calls to the older __sync_* functions with the new atomic_*
  standard ones
* use the _Atomic type modifier as required with stdatomic.h functions
* add dependency for gem_create on libatomic

Signed-off-by: Guillaume Tucker <guillaume.tucker@collabora.com>
Reviewed-by: Simon Ser <simon.ser@intel.com>