ion: fix bugs in cma heap

Implement ion_cma_unmap_kernel, ion will call it unconditionally.
Use correct gfp flags when calling dma_alloc_coherent so it doesn't
try to use atomic DMA memory.
Check for invalid alignment when allocating.
Reject cached allocations - the cpu address returned by
dma_alloc_coherent is always going to be an uncached mapping, so
map_kernel will not see data written by a cached userspace mapping.

Change-Id: I2ea03f28fae3749f6de0b89700b69da3845926ea
Signed-off-by: Colin Cross <ccross@android.com>