qemu script: explicitly set 32 bit depth for x86-64

qemu script: explicitly set 32 bit depth for x86-64

This patch is the same as 6c22c591374d258228f74814cded34a24b4bf2d3,
but for x86-64 targets which exhibit the same problem.

Qemu update from 1.2 to 1.4 now allows for 16bit depth in guests,
whereby previously only 32bit depth was supported. However,
the new support is broken, so we force 32bit depth in all cases.

Signed-off-by: Alexandru DAMIAN <alexandru.damian@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>