drm/i915/dp: Fix DSC line buffer depth programming

Fix the calculation of the DSC line buffer depth. This is limited both
by the source's and sink's maximum line buffer depth, but the former one
was not taken into account. On all Intel platform's the source's maximum
buffer depth is 13, so the overall limit is simply the minimum of the
source/sink's limit, regardless of the DSC version.

This leaves the DSI DSC line buffer depth calculation as-is, trusting
VBT.

On DSC version 1.2 for sinks reporting a maximum line buffer depth of 16
the line buffer depth was incorrectly programmed as 0, leading to a
corruption in color gradients / lines on the decompressed screen image.

Cc: [email protected]
Reviewed-by: Ankit Nautiyal <[email protected]>
Reviewed-by: Manasi Navare <[email protected]>
Acked-by: Maarten Lankhorst <[email protected]>
Signed-off-by: Imre Deak <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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