disp: msm: sde: allow rect1 only configuration

This change allows the configuration where virtual plane is
staged without staging the corresponding master plane. Hw allows
such configuration and additionally during shell stop cases,
framework removes each staged plane one by one which requires
this configuration to be supported.

Change-Id: I85ce571dbbc148c069c2cf35cba55ad740d52b5d
Signed-off-by: Abhijit Kulkarni <kabhijit@codeaurora.org>