drm/exynos: workaround to permit O_RDWR of dma-buf

drm/exynos: workaround to permit O_RDWR of dma-buf

It is possible for userspace to mmap() directly by the commit
bfe981a09528 ("drm: prime: Honour O_RDWR during prime-handle-to-fd")
but userspace using DRM_IOCTL_PRIME_HANDLE_TO_FD should be modified as
adding DRM_RDWR flag.

This patch permits O_RDWR of dma-buf as workaround until all tizen
kernel applies the commit bfe981a09528 ("drm: prime: Honour O_RDWR
during prime-handle-to-fd") userspace uses DRM_RDWR flag on
DRM_IOCTL_PRIME_HANDLE_TO_FD to mmap() dma-buf directly.

Change-Id: I29e86e98cece1fb84af35b2333c998525a94a381
Signed-off-by: Joonyoung Shim <jy0922.shim@samsung.com>