msm: kgsl: Consolidate the contiguous memory allocation functions

We have competing contiguous memory allocation functions that
serve the same purpose. Use CMA for all our contiguous needs
and factor out the legacy code.

Change-Id: Ic0dedbad9965a456b24bc78e1f360242568f15bb
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>