gpu: ion: Use alloc_pages instead of vmalloc from the system heap

With this change the ion_system_heap will only use kernel address
space when the memory is mapped into the kernel (rare case).

Signed-off-by: Rebecca Schultz Zavin <rebecca@android.com>
[jstultz: modified patch to apply to staging directory]
Signed-off-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>