omap: Add DRM_RDWR flag to dmabuf export

Allows mmap on dmabuf fd with MAP_SHARED and PROT_WRITE.

This fixes boot failures caused due to mmap() returning error

Signed-off-by: Hemant Hariyani <hemanthariyani@ti.com>
[picked and updated commitmsg from http://git.ti.com/cgit/cgit.cgi/android/external-libdrm.git/

]
Bug: 74437007
Change-Id: I8ed3ed3ecb8a5005b3aa69a852e3c0f29ef9f43d
Signed-off-by: Praneeth Bajjuri <praneeth@ti.com>
Signed-off-by: Alistair Strachan <astrachan@google.com>