msm: kgsl: Clean up the halt on pagefault path

Clean up a number of pitfalls involved with enabling and using
the halt on pagefault option:

 - Set the default pagetable in the user IOMMU context
 - Program the IOMMU immediately if the pagefault policy changes
   while the GPU is active
 - Don't run the crash dumper for a snapshot that follows a halt
   on pagefault (since the IOMMU is still halted)

Change-Id: Ic0dedbad3bdc8c51cefa6607a0f69da5157a838e
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>