drm/exynos: Remove unused vma field of exynos_drm_gem_obj

The field 'vma' of 'exynos_drm_gem_obj' structure was introduced in
2a3098ff

 ("drm/exynos: add userptr feature for g2d module") but is
not referenced anywhere.

One instance of 'exynos_drm_gem_obj' may be mapped to multiple
user-space VMAs so 'vma' field does not look useful anyway.

Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>