minigbm: add new GBM_BO_IMPORT_FD_PLANAR

minigbm: add new GBM_BO_IMPORT_FD_PLANAR

This is helpful for importing multiplanar formats (see b/31999328)
and importing AFBC format modifiers.

BUG=chrome-os-partner:56407
TEST=drm-tests null_platform_test with AFBC support
     test_that -b cyan $IP1 graphics_Gbm to check for regressions

Change-Id: If7ed9b54e18069ca69b2dbda2b01d59ce58ebeef
Reviewed-on: https://chromium-review.googlesource.com/401439
Commit-Ready: Gurchetan Singh <gurchetansingh@chromium.org>
Tested-by: Gurchetan Singh <gurchetansingh@chromium.org>
Reviewed-by: Stéphane Marchesin <marcheu@chromium.org>
Reviewed-by: Daniele Castagna <dcastagna@chromium.org>