Fix Display White Balance color temperature on enable

Currently the ambient-to-display color temperature spline is applied to
all color temperature values, including the placeholder value -1.0 that
is used when DWB is first enabled. When this happens, display color
temperature is first set to its lowest value, and is then set to a
meaningful value when the sensor updates its first reading. This
behavior is incorrect.

To fix this, check for a valid color temperature value
before applying the spline.

Bug 130137407
Test: manual

Change-Id: I67a06504e499411fd34e05569e9cd0cbc9f81df1