drm/msm/dsi-staging: fix t_clk_pre in high dsi clock use case

The t_clk_pre extend register used to configure the
resolution of the t_clk_pre setting. Without this set, the
duration of the t_clk_pre would be half of what is needed.
This change adds the provision to configure this register.

Change-Id: Iac12ac3da949e042fd560fa3945a97555d1283e6
Signed-off-by: Rajeev Nandan <rajeevny@codeaurora.org>