msm: mdss: avoid clocks on if panel is not on for command mode

There are cases where vsync enable may happen before panel has been
turned on. In this case MDP clocks and specially DSI clocks shouldn't
get turned on if the panel is not ready yet. Add a check to ensure
clocks are not turned on and wait for panel to be turned on later on,
vsync events will be enabled then.

Change-Id: I522a1ecafc3949582e049d247a443725672d2d47
Signed-off-by: Adrian Salido-Moreno <adrianm@codeaurora.org>
(cherry picked from commit ed3881951d0930b2bcd5f41ae2ffc06c9fada0b7)