This fixes a compiler warning treated as an error when building for
32-bit architectures since their pointer size does not match the size
of drm_i915_gem_context_param.value which is 64 bits:

  CC       i915/gem_ctx_sseu.o
  i915/gem_ctx_sseu.c: In function ‘test_ggtt_args’:
  i915/gem_ctx_sseu.c:384:9: error: cast to pointer from integer of different size [-Werror=int-to-pointer-cast]
    munmap((void *)arg.value, 4096);

It was found while building for arm with gcc 6.3.0 and I suspect the
same problem would arise for i386 or other 32-bit architectures.  The
uintptr_t type is by definition an unsigned integer of the same length
as a pointer on a given architecture, so this should fix the problem
for all architectures up to 64 bits.
Signed-off-by: Guillaume Tucker <guillaume.tucker@collabora.com>