hwc: Use maximum mixer width supported to identify split display

MDP pipe width should be used to find the number of overlay
pipes needed for configuring a hwc layer. Split display should
use the max mixer width supported by the H/W to check against display
resolution to detect split displays.

Change-Id: I76f97b33ea4a11270a6e33a3942bce0c4b4bae1f
1 file changed