drm/exynos: dsi: Fix the parse_dt function

The dsi + panel is a parental relationship, so OF grpah is not needed.
Therefore, the current dsi_parse_dt function will throw an error,
because there is no linked OF graph for the case fimd + dsi + panel.

Parse the Pll burst and esc clock frequency properties in dsi_parse_dt()
and create a bridge_node only if there is an OF graph associated with dsi.

Signed-off-by: Hoegeun Kwon <hoegeun.kwon@samsung.com>
Reviewed-by: Andrzej Hajda <a.hajda@samsung.com>
Reviewed-by: Andi Shyti <andi.shyti@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>