ANDROID: cma_get_first_virtzone_base: fix !CONFIG_CMA error

cma_get_first_virtzone_base() was not defined if CONFIG_CMA is not set.

Define it to return 0 (no-op) if !CONFIG_CMA.

Bug: 400651191
Bug: 313807618
Test: tools/bazel run --lto=none
           //common:kernel_aarch64_microdroid_16k_dist
Change-Id: I9083ece26d60cc967baf93e8e29b16d6b0901a15
Signed-off-by: Kalesh Singh <kaleshsingh@google.com>