input: touchscreen: bu21150: move AFE control signals to optional

AFE PWR and DISP VSN control signals currently in driver are
required. But on some platform, they are not needed. Update
driver to address this need.

Change-Id: Ib09ad708d79554f755f4846a3c01d8c0156ded89
Signed-off-by: Chun Zhang <chunz@codeaurora.org>