minigbm: More sophisticated gbm_bo_map/gbm_bo_unmap

minigbm: More sophisticated gbm_bo_map/gbm_bo_unmap

We previously added the gbm_bo_map/gbm_bo_unmap (see CL:393927)
entry points since we wanted to do driver-specific detiling during
screenshot capture tests.  We ignored most the parameters and mapped
the entire buffer.  This CL adds the ability to:

1) Return the starting address within a byte given a specific x, y
   in the buffer.

2) Handle the case where there are more than one kernel buffers
   per buffer object.  Currently, only the Exynos driver would use
   this capability.

BUG=chromium:653284
TEST=Ran cros_gralloc with modified code

CQ-DEPEND=CL:393927

Change-Id: I19d75d2f16489c0184e96305fb643f18477e1cdb
Reviewed-on: https://chromium-review.googlesource.com/395066
Commit-Ready: Gurchetan Singh <gurchetansingh@chromium.org>
Tested-by: Gurchetan Singh <gurchetansingh@chromium.org>
Reviewed-by: Stéphane Marchesin <marcheu@chromium.org>