leds: leds-qpnp: avoid calling pwm_disable when scaling leds' brightness

The LPG hardware supports dynamically changing the duty cycle
without disabling the output. So pwm_disable calling can be
removed when setting nonzero brightness level, which avoid the
delays of the PWM wave output when changing PWM configuration.

CRs-Fixed: 622683
Change-Id: I8c5bdef1992c41f73fe16cce615b2d1d6dad7775
Signed-off-by: Xu Kai <kaixu@codeaurora.org>