ion: don't use phys_to_page or __phys_to_pfn

phys_to_page and __phys_to_pfn don't exist on all platforms.
Use a combination of pfn_to_page, PFN_DOWN, page_to_pfn, and
virt_to_page to get the same results.

Signed-off-by: Colin Cross <ccross@android.com>
Signed-off-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>