drm: msm: sde: Allocate last command buffers per control

Each display can be assigned a control in the SDE hardware block. Using
a single last command buffer for all controls can create race condition
if each display is running in separate thread. Change allocates one last
command buffer per display.

Change-Id: I0652e58018176647d8c06ab0c1502c785bcbc9b9
Signed-off-by: Gopikrishnaiah Anandan <agopik@codeaurora.org>