msm: display: Making vsync_clk optional in dt parsing

vsync_clk is optional for non-smart panels, so need to make its check
optional in device tree parsing to prevent probe failure.

Change-Id: Ic364c1c1ecde07c44c13f7925bc85e2278c1de2a
Signed-off-by: Zohaib Alam <zalam@codeaurora.org>