drm/msm/dpu: Fall back to a single DSC encoder (1:1:1) on small SoCs

Some SoCs such as SC7280 (used in the Fairphone 5) have only a single
DSC "hard slice" encoder.  The current hardcoded use of 2:2:1 topology
(2 LM and 2 DSC for a single interface) make it impossible to use
Display Stream Compression panels with mainline, which is exactly what's
installed on the Fairphone 5.

By loosening the hardcoded `num_dsc = 2` to fall back to `num_dsc =
1` when the catalog only contains one entry, we can trivially support
this phone and unblock further panel enablement on mainline.  A few
more supporting changes in this patch ensure hardcoded constants of 2
DSC encoders are replaced to count or read back the actual number of
DSC hardware blocks that are enabled for the given virtual encoder.
Likewise DSC_MODE_SPLIT_PANEL can no longer be unconditionally enabled.

Cc: Luca Weiss <luca.weiss@fairphone.com>
Signed-off-by: Marijn Suijten <marijn.suijten@somainline.org>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Tested-by: Luca Weiss <luca.weiss@fairphone.com>
Reviewed-by: Jessica Zhang <quic_jesszhan@quicinc.com>
Tested-by: Danila Tikhonov <danila@jiaxyga.com>
Patchwork: https://patchwork.freedesktop.org/patch/633318/
Link: https://lore.kernel.org/r/20250122-dpu-111-topology-v2-1-505e95964af9@somainline.org
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>

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