hwc: src split: Fix unwanted scaling with H-flip

Source split splits the crop and position into half, but if the
widths are odd and the left, right crops are swapped for H-flip, it
induces a scaling that is unaccounted for while allocating pipes.
This could lead to DMA pipes having scaling.

To fix this, the patch manipulates position width, if a swap happens.

Change-Id: I1b3ed582740fd49e651407be422820f60fe59cc6