minigbm: Added support for multiplane import

minigbm: Added support for multiplane import

Gralloc requires flexible YUV formats (YV12, NV12) to be importable.
This change modifies our internal import API to support this.

TEST=ran graphics_Gbm
BUG=chromium:616275
CQ-DEPEND=CL:373048

Change-Id: I4100e1c1639828e4adf08764b45fe5a44b7078a3
Reviewed-on: https://chromium-review.googlesource.com/374162
Commit-Ready: Gurchetan Singh <gurchetansingh@chromium.org>
Tested-by: Gurchetan Singh <gurchetansingh@chromium.org>
Reviewed-by: Stéphane Marchesin <marcheu@chromium.org>