cros_gralloc: move cflags select into minigbm_defaults build rule

from the current libgbm build rule, so that one minigbm build rule
would work for all platforms based on the minigbm platform
soong config varible which is set in the device repository.

Test: boot to homescreen with both msm and generic minigbm
      allocator and mapper versions

Change-Id: I939730626eb478f699b1b099054446f25bb7289c
Signed-off-by: Yongqin Liu <yongqin.liu@linaro.org>
Signed-off-by: Amit Pundir <amit.pundir@linaro.org>
Suggested-by: Jason Macnak <natsu@google.com>