backlight: qcom-spmi-wled: Initialize mutex before it can be used

Initialize mutex lock early so that it can be used properly when
an interrupt handler is called right after registering for it.

Change-Id: Ie2ec354e668d4681c9b7e01751e9dd426a295aa4
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>