drm/msm/dpu: fix mixer number counter on allocation

Current code only supports usage cases with one pair of mixers at
most. To support quad-pipe usage case, two pairs of mixers need to
be reserved. The lm_count for all pairs is cleared if a peer
allocation fails in current implementation. Reset the current lm_count
to an even number instead of completely clearing it. This prevents all
pairs from being cleared in cases where multiple LM pairs are needed.

Signed-off-by: Jun Nie <jun.nie@linaro.org>
Reviewed-by: Jessica Zhang <quic_jesszhan@quicinc.com>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Patchwork: https://patchwork.freedesktop.org/patch/675400/
Link: https://lore.kernel.org/r/20250918-v6-16-rc2-quad-pipe-upstream-4-v16-1-ff6232e3472f@linaro.org
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>