drm/i915/dp: Return min bpc supported by source instead of 0

Currently, intel_dp_dsc_max_src_input_bpc can return 0 for platforms not
supporting DSC, which could theoretically cause issues in clamp()
due to a low limit being greater than the high limit.

Instead, return the minimum bpc supported by the source to prevent
such issues.

Reported-by: Linux Kernel Functional Testing <lkft@linaro.org>
Closes: https://lore.kernel.org/all/CA+G9fYtNfM399_=_ff81zeRJv=0+z7oFJfPGmJgTp6yrJmU+1w@mail.gmail.com/


Fixes: 160672b8 ("drm/i915/dp: Use clamp for pipe_bpp limits with DSC")
Cc: Suraj Kandpal <suraj.kandpal@intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Reviewed-by: Suraj Kandpal <suraj.kandpal@intel.com>
Tested-by: Chaitanya Kumar Borah <chaitanya.kumar.borah@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250131041342.3086716-1-ankit.k.nautiyal@intel.com


Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>