regulator: qpnp-labibb: do not disable IBB when entering TTW mode

Currently when entering touch-to-wakeup (TTW) mode, IBB module
is disabled as part of the regulator_disable sequence before
applying the settings needed for SWIRE control in TTW mode.
This causes adversary effects such as droop in the VDISN
voltage rail.

To avoid this, as per the hardware documentation, do not disable
IBB module through IBB_ENABLE_CTL when LABIBB regulators are
disabled and entering TTW mode. Instead, just enter TTW mode and
mark the regulators disabled. Normal mode settings will be
applied when the regulators are enabled again as before.

While at it, add a centralized function qpnp_ibb_mode() to write
to IBB_ENABLE_CTL register.

CRs-Fixed: 952847
Change-Id: I74c6ba411091d146ab5719c2d72d6385fa90af21
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>