video: rockchip: dw_mipi_dsi: Fix external phy existence check

&priv->phy is always true. Compiler warns about this loudly.

Use a propper check for phy device allocation. Without this fix
using this driver with SoC that doesn't use external phy (eg. RK3399)
doesn't work.

Signed-off-by: Ondrej Jirman <megi@xff.cz>