drm/sde: always use two hw physical pipes as defined in dtsi

When two hardware physical pipes are defined in dtsi for one virtual
plane, driver should always use them and calculate correct source
and destination rectangles.

CRs-Fixed: 1084262
Change-Id: I0bfa69316889e5b3c35c2ca2c1884a19092f364c
Signed-off-by: Jin Li <jinl@codeaurora.org>