minigbm: add a default (*bo_unmap) implementation

minigbm: add a default (*bo_unmap) implementation

The (*bo_unmap) function is responsible for unmapping the buffer
and cleaning up any data allocated during (*bo_map). Previously,
we called munmap in drv_bo_unmap(), which caused some confusion.
This method is cleaner.

BUG=chromium:764871
TEST=emerge-betty minigbm

Change-Id: I4dc20cd6b15e79bce21d33f03ebc84480c582981
Reviewed-on: https://chromium-review.googlesource.com/671693
Commit-Ready: Gurchetan Singh <gurchetansingh@chromium.org>
Tested-by: Gurchetan Singh <gurchetansingh@chromium.org>
Reviewed-by: Joe Kniss <djmk@google.com>
Reviewed-by: Tomasz Figa <tfiga@chromium.org>