video: rockchip: dw_mipi_dsi: Use generic_phy_valid() helper

The documentation for struct phy state that "The content of the
structure is managed solely by the PHY API and PHY drivers".

Change to use the generic_phy_valid() helper to check if phy is valid.

Fixes: b7d8d403 ("video: rockchip: dw_mipi_dsi: Fix external phy existence check")
Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>