drm/msm/dpu: remove DPU_DSPP_IGC handling in dspp flush

drm/msm/dpu: remove DPU_DSPP_IGC handling in dspp flush

Inverse gamma correction blocks (IGC) are not used today so lets
remove the usage of DPU_DSPP_IGC in the DSPP flush to make it easier
to remove IGC from the catalog.

We can add this back when IGC is properly supported in DPU with
one of the standard DRM properties.

changes in v3:
	- minor change dspp -> DSPP in commit text

Signed-off-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Marijn Suijten <marijn.suijten@somainline.org>
Patchwork: https://patchwork.freedesktop.org/patch/534724/
Link: https://lore.kernel.org/r/20230428223646.23595-2-quic_abhinavk@quicinc.com
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>