Set bounds from the array when iterating mmaps.

The mmap allocation code in vp8_dx_iface.c was inconsistent.
The static array vp8_mem_req_segs defines two descriptors,
but only the first is real. The second is a sentinel and
isn't actually allocated, so vpx_codec_alg_priv is declared
with mmaps[NELEMENTS(vp8_mem_req_segs)-1]. Some functions
use this reduced upper bound when iterating though the mmap
array, but these two functions did not.

Instead, this commit calls NELEMENTS(...->mmaps) to directly
query the bounds of the dereferenced array.

This fixes an array-bounds warning from gcc 4.6 on
vp8_xma_set_mmap.

Change-Id: I918e2721b401d134c1a9764c978912bdb3188be1