intel: fix check for 48b ppgtt support

intel: fix check for 48b ppgtt support

The previous logic of the supports_48b_addresses wasn't actually
checking if i915.ko was running with full_48bit_ppgtt. The ENOENT
it was checking for was actually coming from the invalid context
id provided in the test execbuffer.  There is no path in the
kernel driver where the presence of
EXEC_OBJECT_SUPPORTS_48B_ADDRESS leads to an error.

Instead, check the default context's GTT_SIZE param for a value
greater than 4 GiB

v2 (Ken): Fix in i965 as well.
v3 Check GTT_SIZE instead of HAS_ALIASING_PPGTT (Chris Wilson)

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>