drm/msm/sde: Check for cur_master NULL dereference

Check if cur_master encoder exists before using the variable in
sde encoder. This will avoid possible NULL dereferences.

Change-Id: I003386cba392a0027f7e9e8441fd4671b57b9a03
Signed-off-by: Nilaan Gunabalachandran <ngunabal@codeaurora.org>