drm/exynos: Always initialize mapping in exynos_drm_register_dma()

drm/exynos: Always initialize mapping in exynos_drm_register_dma()

In certain randconfigs, clang warns:

drivers/gpu/drm/exynos/exynos_drm_dma.c:121:19: warning: variable
'mapping' is uninitialized when used here [-Wuninitialized]
                priv->mapping = mapping;
                                ^~~~~~~
drivers/gpu/drm/exynos/exynos_drm_dma.c:111:16: note: initialize the
variable 'mapping' to silence this warning
                void *mapping;
                             ^
                              = NULL
1 warning generated.

This occurs when CONFIG_EXYNOS_IOMMU is enabled and both
CONFIG_ARM_DMA_USE_IOMMU and CONFIG_IOMMU_DMA are disabled, which makes
the code look like

  void *mapping;

  if (0)
    mapping = arm_iommu_create_mapping()
  else if (0)
    mapping = iommu_get_domain_for_dev()

  ...
  priv->mapping = mapping;

Add an else branch that initializes mapping to the -ENODEV error pointer
so that there is no more warning and the driver does not change during
runtime.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Inki Dae <inki.dae@samsung.com>