minigbm: add support for HAL_PIXEL_FORMAT_RGBA_FP16

minigbm: add support for HAL_PIXEL_FORMAT_RGBA_FP16

This is needed to support the following CTS test:

android.graphics.cts.BitmapColorSpaceTest#test16bitHardware

There have been some rumblings about adding 64-bit formats to <drm_fourcc.h>:

https://lists.freedesktop.org/archives/intel-gvt-dev/2017-July/001469.html

However, nothing has landed, so let's just define our own format
for the time being.

BUG=b:77973662
TEST=Compile for kevin and kevin-arcnext
     Unfortunately, my P setup refuses to work, so let the lab
     test.

Change-Id: I1fea16400ba6632a8ef17105e27bc7799d2af515
Reviewed-on: https://chromium-review.googlesource.com/1029355
Commit-Ready: Gurchetan Singh <gurchetansingh@chromium.org>
Tested-by: Gurchetan Singh <gurchetansingh@chromium.org>
Reviewed-by: Gurchetan Singh <gurchetansingh@chromium.org>