video: exynos_mipi_dsi: Remove unnecessary NULL check

'dsim' is allocated and checked for NULL in the probe function.
Hence this check is redundant. This cleanup also fixes a potential NULL
pointer dereference error when dsim which is NULL references its member
in the error print message.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Acked-by: Inki Dae <inki.dae@samsung.com>
Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>