libdrm: amdgpu: Adding DRM_RDWR flag in amdgpu_bo_export

Currently while exporting prime handle to fd read write access is
not granted. mmap fails because of this. mmap was not supported on
prime initially.
Here is link to related discussion
https://lists.freedesktop.org/archives/dri-devel/2017-February/131840.html



Adding the DRM_RDWR flag in amdgpu_bo_export to support mmap.

Signed-off-by: Satyajit <satyajit.sahu@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>