pixman: check neon support via TUNE_FEATURES, not the _armv7a over-ride

pixman: check neon support via TUNE_FEATURES, not the _armv7a over-ride

Neon support is optional in armv7a so the _armv7a over-ride is not
the best way to determine whether or not the target supports neon.

Since pixman will always use neon in preference to 'simd' (ie VFPv2)
if it can, it's safe to disable the simd routines if the target is
known to support neon.

(From OE-Core rev: ad8337a127a8af321396b78a1cf331b54e4e0515)

Signed-off-by: Andre McCurdy <armccurdy@gmail.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>