drm/exynos: Remove unused vma field of exynos_drm_gem_obj

The field 'vma' of 'exynos_drm_gem_obj' structure was introduced in
2a3098ff6c21 ("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 <[e-mail]>
Signed-off-by: Inki Dae <[e-mail]>

drivers/gpu/drm/exynos/exynos_drm_gem.h

···
  *	or at framebuffer creation.
  * @size: size requested from user, in bytes and this size is aligned
  *	in page unit.
- * @vma: a pointer to vm_area.
  * @flags: indicate memory type to allocated buffer and cache attruibute.
  *
  * P.S. this object would be transferred to user as kms_bo.handle so
···
 	struct drm_gem_object		base;
 	struct exynos_drm_gem_buf	*buffer;
 	unsigned long			size;
-	struct vm_area_struct		*vma;
 	unsigned int			flags;
 };