minigbm: ensure drm_prime_handle struct is correctly initialized

minigbm: ensure drm_prime_handle struct is correctly initialized

the 'prime_handle' variable has a 'flags' and the kernel code for
DRM_IOCTL_PRIME_FD_TO_HANDLE will fail if 'flags' contains something
else than DRM_CLOEXEC or 0.
Right now, the 'flags' field is uninitialized and DRM initialization
fails on some VM configs with kernel 4.4.

BUG=chromium:591626
TEST=run null_platform_test and Chrome browser using the arm64-llvmpipe
overlay and kernel 4.4 on QEMU VM.

Change-Id: Ieac8eb280fe14f1e268f8869dcc9fe46c1bb9599
Reviewed-on: https://chromium-review.googlesource.com/337970
Commit-Ready: Stéphane Marchesin <marcheu@chromium.org>
Tested-by: Stéphane Marchesin <marcheu@chromium.org>
Reviewed-by: Stéphane Marchesin <marcheu@chromium.org>