msm: kgsl: Stop using KGSL_MEMDESC_CONTIG

KGSL_MEMDESC_CONTIG is only used for global GPU allocations but there are
no longer any global GPU objects that _need_ contiguous memory. Remove
all usage of it and transition the one remaining multi-page user
(memstore) to the paged allocator instead.

Change-Id: Ic0dedbadda417a1d4103e165c13d1debb3316b72
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>