minigbm: introduce drv_array

minigbm: introduce drv_array

We want to allow multiple mappings of a single buffer with different
map flags. To do this, we'll need some sort of linked list or dynamic
array of mappings. We already use dynamic arrays in minigbm, so let's
centralize that functionality and use it.

BUG=chromium:764871
TEST=emerge-kevin {minigbm, arc-cros-gralloc}

Change-Id: I2b391d6e8e690eac6368b1404a806b2bfbbcdf44
Reviewed-on: https://chromium-review.googlesource.com/758145
Commit-Ready: Gurchetan Singh <gurchetansingh@chromium.org>
Tested-by: Gurchetan Singh <gurchetansingh@chromium.org>
Reviewed-by: Stéphane Marchesin <marcheu@chromium.org>