drm/msm/dp: reject YUV420-only modes without VSC SDP support

DP conveys YUV 420 colorimetry through a VSC SDP. A sink that advertises
a mode as YUV-420-only therefore cannot be driven at all unless the panel
supports VSC SDP, yet msm_dp_bridge_mode_valid() only used the VSC SDP
capability to decide whether to halve the pixel clock, otherwise letting
such modes through to be validated (and possibly accepted) at the full
RGB clock the sink cannot display.

Reject 420-only modes with MODE_NO_420 when the panel does not support
VSC SDP. With those modes filtered out, being a 420-only mode implies VSC
SDP support, so the YUV-420 test reduces to drm_mode_is_420_only(): drop
msm_dp_is_yuv_420_enabled() and call the DRM helper directly at its two
callers (the DPU encoder already has the connector from the atomic state).

Fixes: df9cf852ca30 ("drm/msm/dp: account for widebus and yuv420 during mode validation")
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
Patchwork: https://patchwork.freedesktop.org/patch/741713/
Link: https://lore.kernel.org/r/20260722-drm-msm-display-interface-v1-1-368c10fe62fd@oss.qualcomm.com

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