msm: kgsl: Workaround for missing pgprot macros

The macros pgprot_writethroughcache() and pgprot_writebackcache()
do not exist in arm64. Default caching policy is writeback and
writethroughcache is not supported by the hardware so tell the
user that we are using writeback instead.

Change-Id: Ifa67ec8d87a3b4dc98504084141cc105abb71a09
Signed-off-by: Shrenuj Bansal <shrenujb@codeaurora.org>