disp: msm: sde: remove clearing cur_master in encoder enable function

SDE IRQ callback can run in parallel thread to modeset after removing
pp_done wait before pre_modeset.
If cur_master is cleared in encoder enable function and irq callback
is triggered at the same time, the irq callback could not be handled
properly as cur_master is NULL.So remove clearing cur_master in
encoder enable function to avoid the race condition between modeset
and irq callback.

Change-Id: I2059c699a68838b3c9f6a7dd658a35f178b18c42
Signed-off-by: Lei Chen <quic_chenlei@quicinc.com>
Signed-off-by: Jayaprakash Madisetty <quic_jmadiset@quicinc.com>