drm/exynos: remove function convert_to_vm_err_msg

The convert_to_vm_err_msg is called just once by exynos_drm_gem_fault,
so it's simple not to use the function.

Signed-off-by: Joonyoung Shim <jy0922.shim@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>