msm: mdss: dp: correctly ref count the pll resources

This fixes the issue where the dp pll resource was not correctly
keeping a refcount on the pll resource. This will fix the bootup
warning when both DSI and DP are enabled.

Crs-Fixed: 1088737
Change-Id: I19f8eef7f664a58cac1a082b8195e48c52613c5d
Signed-off-by: Siddhartha Agrawal <agrawals@codeaurora.org>