drm/msm/dpu: enable sourcesplit for sc7180/sc7280

According to the vendor dts files, both sc7180 and sc7280 support the
source split mode (using two LMs for a single output). Change these two
platforms to use MIXER_SDM845_MASK, which includes
DPU_MIXER_SOURCESPLIT. Rename MIXER_SC7180_MASK to MIXER_QCM2290_MASK,
since this platform doesn't seem to support source split mode.

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Patchwork: https://patchwork.freedesktop.org/patch/518479/
Link: https://lore.kernel.org/r/20230116034435.569512-1-dmitry.baryshkov@linaro.org
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>