drm/exynos: Add fallback option to get non physically contiguous memory for fb

While trying to get boot-logo up on exynos5420 SMDK which has eDP panel
connected with resolution 2560x1600, following error occured even with
IOMMU enabled:
[0.880000] [drm:lowlevel_buffer_allocate] *ERROR* failed to allocate buffer.
[0.890000] [drm] Initialized exynos 1.0.0 20110530 on minor 0

To address the cases where physically contiguous memory MAY NOT be a
mandatory requirement for fb, the patch adds a feature to get non physically
contiguous memory for fb if physically contiguous memory allocation fails
and if IOMMU is supported.

Signed-off-by: Vikas Sajjan <vikas.sajjan@linaro.org>
Signed-off-by: Arun Kumar <arun.kk@samsung.com>
Reviewed-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>