libdrm: Use mmap64 instead of __mmap2

__mmap2 isn't supported on all platforms, mmap64 is the right way
to do this in android.

Also folds in a fix from Stéphane Marchesin <marcheu@chromium.org>

Cc: Sean Paul <seanpaul@chromium.org>
Cc: Alistair Strachan <astrachan@google.com>
Cc: Marissa Wall <marissaw@google.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
[jstultz: Folded in Stéphane's fix]
Signed-off-by: John Stultz <john.stultz@linaro.org>