drm/msm/dsi: drop gdsc regulator handling

drm/msm/dsi: drop gdsc regulator handling

None of supported devies uses "gdsc" regulator for DSI. GDSC support is
now implemented as a power domain. Drop old code and config handling
gdsc regulator requesting and enabling.

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Link: https://lore.kernel.org/r/20210701000015.3347713-1-dmitry.baryshkov@linaro.org
Reviewed-by: Abhinav Kumar <abhinavk@codeaurora.org>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>