drm/msm/mdp5: remove less than 0 comparison for unsigned value

pipe is an unsigned int and less than zero comparison for unsigned
values is always false.

Detected using the following cocci script:

@@
unsigned int i;
@@
* i < 0

Signed-off-by: Aishwarya Pant <aishpant@gmail.com>
Reviewed-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20171010184207.iv3dinrtwvbv7fei@aishwarya