staging: ion: Fix ion_cma_heap allocations

In trying to add support for drm_hwcomposer to HiKey,
I've needed to utilize the ION CMA heap, and I've noticed
problems with allocations on newer kernels failing.

It seems back with 204f6722 ("ion: Use CMA APIs directly"),
the ion_cma_heap code was modified to use the CMA API, but
kept the arguments as buffer lengths rather then number of pages.

This results in errors as we don't have enough pages in CMA to
satisfy the exaggerated requests.

This patch converts the ion_cma_heap CMA API usage to properly
request pages.

It also fixes a minor issue in the allocation where in the error
path, the cma_release is called with the buffer->size value which
hasn't yet been set.

Cc: Laura Abbott <labbott@redhat.com>
Cc: Sumit Semwal <sumit.semwal@linaro.org>
Cc: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Cc: Archit Taneja <architt@codeaurora.org>
Cc: Greg KH <gregkh@linuxfoundation.org>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Dmitry Shmidt <dimitrysh@google.com>
Cc: Todd Kjos <tkjos@google.com>
Cc: Amit Pundir <amit.pundir@linaro.org>
Fixes: 204f6722

 ("staging: android: ion: Use CMA APIs directly")
Acked-by: Laura Abbott <labbott@redhat.com>
Signed-off-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>