drm/msm/dp: Remove global g_dp_display variable

drm/msm/dp: Remove global g_dp_display variable

As the Qualcomm DisplayPort driver only supports a single instance of
the driver the commonly used struct dp_display is kept in a global
variable. As we introduce additional instances this obviously doesn't
work.

Replace this with a combination of existing references to adjacent
objects and drvdata.

Reviewed-by: Abhinav Kumar <abhinavk@codeaurora.org>
Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Link: https://lore.kernel.org/r/20211016221843.2167329-2-bjorn.andersson@linaro.org
Signed-off-by: Rob Clark <robdclark@chromium.org>