msm: pil: Don't call ion_free() multiple times

We don't want to call ion_free() on the priv->region pointer if
the first call to pil_boot() failed but got far enough to
allocate an ion region. Assign the pointer to NULL so that we
don't call ion_free() twice.

Otherwise we hit the BUG in ion.c.

kernel BUG at drivers/gpu/ion/ion.c:498!
Internal error: Oops - BUG: 0 [#1] PREEMPT SMP ARM
Modules linked in: adsp_loader
CPU: 0 Tainted: G W (3.4.0+ #4)
PC is at ion_free+0x18/0xa0
LR is at pil_release_mmap+0x20/0x60

Change-Id: Ief8c61335212313f1727c26b2b30ca6b66b7f36f
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>