msm: kgsl: Allocate the GMU cacheable range in one chunk

This solves the problem of poor utilization of the GMU
cacheable memory range as a result of fragmentation.
Allocating it in one chunk avoids fragmentation and also
gives us the flexibility to carve out regions as necessary
through the scatter file without requiring any kgsl side
changes.

Change-Id: I42a50dbcad039cd0788df6612898cfaaf6426f22
Signed-off-by: Carter Cooper <ccooper@codeaurora.org>