benchmarks/gem_wsim: Fix mmap for discrete graphics cards

It appears that 'gem_mmap__wc' doesn't work for discrete graphics cards
and 'I915_MMAP_OFFSET_FIXED' is needed instead.
Adopt the mapping approach from 'lib/igt_dummyload'.

v2: Correct brace balance (Kamil)
    Remove redundant 'ptr' variable

Signed-off-by: Marcin Bernatowicz <marcin.bernatowicz@linux.intel.com>
Reviewed-by: Kamil Konieczny <kamil.konieczny@linux.intel.com>