drm/bridge: dw-mipi-dsi: Use kmemdup cf. kmalloc+memcpy

kmemdup can be used instead of kmalloc+memcpy. Replace an occurrence of
this pattern.

Issue identified with Coccinelle.

Signed-off-by: Alex Dewar <alex.dewar90@gmail.com>
Acked-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200909190213.156302-1-alex.dewar90@gmail.com