clk: qcom: mdss: fix blank / unblank for DSI 28nm pll

During unblank write the cached vreg config values of pll.
For splash enabled vreg is configured in Direct mode,
which needs to be configured back to In-Direct mode
during un-blank.

Change-Id: I739a8fe0b3d643bdf25b58fa2c17dadd19f6bdb7
Signed-off-by: Raghavendra Ambadas <rambad@codeaurora.org>