phy: exynos-mipi-video: Fixup the test for state->regmap

syscon_regmap_lookup_by_phandle() returns ERR_PTR on error.
Thus don't use null test against state->regmap.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>