libdrm: omap: Add DRM_RDWR flag to dmabuf export

Allows mmap on dmabuf fd with MAP_SHARED and PROT_WRITE.

This fixes boot failures with Android (likely w/ closed source
user-space drivers) that were caused due to mmap() returning
error.

Cc: Sean Paul <seanpaul@chromium.org>
Cc: Alistair Strachan <astrachan@google.com>
Cc: Marissa Wall <marissaw@google.com>
Acked-by: Emil Velikov <emil.velikov@collabora.com>
Signed-off-by: Hemant Hariyani <hemanthariyani@ti.com>
[picked and updated commitmsg from http://git.ti.com/cgit/cgit.cgi/android/external-libdrm.git/

]
Signed-off-by: Praneeth Bajjuri <praneeth@ti.com>
Signed-off-by: Alistair Strachan <astrachan@google.com>
[jstultz: Tweaked commit message]
Signed-off-by: John Stultz <john.stultz@linaro.org>