hwc: Source Split layers always if indicated by driver

Source Split layers always if the driver indicates so by setting
the src_split_always in the capabilities sysfs node.
This is applicable to layers whose width > mixer's width, since
MDP clk rate is dependent on max(mixer_width, layer_width).

This feature can be enabled if the panel height is more than a
certain empirically derived value. Splitting always in those
cases can help with a lower voltage.

Change-Id: If9b33f11ca4132eb77416c26819f7473c65d3b08