msm: mdss: Flush pending clk work before set ctx variable to 0

When clk work function use ctx variable, this variable can be set 0 array
by mdss_mdp_cmd_stop function and it causes kernel panic.
To fix kernel panic of NULL pointer dereference, flush pending clk work
before set ctx variable to 0.

Change-Id: I0206d40ae7e1ab5857feb2e1ed4fea3b74a0852f
Signed-off-by: Han Jun-Yeong <junyeong.han@lge.com>
Signed-off-by: Devin Kim <dojip.kim@lge.com>