drm/msm/dsi: fix 32-bit clang warning

drm/msm/dsi: fix 32-bit clang warning

clang is a little overzealous with warning about a constant conversion
in an untaken branch of a ternary expression:

drivers/gpu/drm/msm/dsi/phy/dsi_phy_7nm.c:975:48: error: implicit conversion from 'unsigned long long' to 'unsigned long' changes value from 5000000000 to 705032704 [-Werror,-Wconstant-conversion]
        .max_pll_rate = (5000000000ULL < ULONG_MAX) ? 5000000000UL : ULONG_MAX,
                                                      ^~~~~~~~~~~~

Rewrite this to use a preprocessor conditional instead to avoid the
warning.

Fixes: ddd71b0deb0d ("drm/msm/dsi: move min/max PLL rate to phy config")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20210514213032.575161-1-arnd@kernel.org
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>