msm: kgsl: Fix NR_KERNEL_MISC_RECLAIMABLE page stats issue

We do call _kgsl_pool_get_page() when pool is empty through
page pool allocation path. This will cause incorrect page
stats accounting for NR_KERNEL_MISC_RECLAIMABLE. Hence Add
a NULL check before page stats to fix it.

Change-Id: I5823177d7e242db25aeca2da5a9ddf6d1982859b
Signed-off-by: Hareesh Gundu <hareeshg@codeaurora.org>