drm: fix DRM_DISPLAY_DP_HELPER dependencies

drm: fix DRM_DISPLAY_DP_HELPER dependencies

Both the exynos and rockchip drivers ran into link failures after
a Kconfig cleanup:

aarch64-linux-ld: drivers/gpu/drm/exynos/exynos_dp.o: in function `exynos_dp_resume':
exynos_dp.c:(.text+0xc0): undefined reference to `analogix_dp_resume'
aarch64-linux-ld: drivers/gpu/drm/exynos/exynos_dp.o: in function `exynos_dp_suspend':
exynos_dp.c:(.text+0xf4): undefined reference to `analogix_dp_suspend'

x86_64-linux-ld: drivers/gpu/drm/rockchip/cdn-dp-core.o: in function `cdn_dp_connector_mode_valid':
cdn-dp-core.c:(.text+0x13a): undefined reference to `drm_dp_bw_code_to_link_rate'
x86_64-linux-ld: cdn-dp-core.c:(.text+0x148): undefined reference to `drm_dp_bw_code_to_link_rate'
x86_64-linux-ld: drivers/gpu/drm/rockchip/cdn-dp-core.o: in function `cdn_dp_check_link_status':
cdn-dp-core.c:(.text+0x1396): undefined reference to `drm_dp_channel_eq_ok'

In both cases, the problem is that ROCKCHIP_CDN_DP and DRM_EXYNOS_DP
are 'bool' symbols that depend on the the 'tristate' DRM_DISPLAY_HELPER
symbol, but end up not working when the SoC specific part is built-in
but the helper is in a loadable module.

Use the same trick that DRM_ROCKCHIP already uses for the EXTCON
dependency and disallow DP support when it would not work.

Fixes: 0323287de87d ("drm: Switch DRM_DISPLAY_DP_HELPER to depends on")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20240404124101.2988099-1-arnd@kernel.org
Signed-off-by: Maxime Ripard <mripard@kernel.org>