msm: kgsl: ignore KGSL_MEMFLAGS_GPUWRITEONLY

This flag doesn't represent anything that any ARM architecture iommu
can actually do. If userspace sets it in the allocation flags, it will
be clear when their ioctl returns and userspace can decide to fail
if necessary.

Change-Id: I956f6ee51e4f2d898b75ae34921459d1d756bd54
Signed-off-by: Jeremy Gebben <jgebben@codeaurora.org>