regulator: qpnp-amoled: Cleanup unused registers

Remove unused register definitions. While at it, convert
qpnp_amoled_read() to static and use it instead of regmap_read().

Change-Id: I390a9c3fd8a8b05b3a0d1d341a3b67e00342e229
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>