surfaceflinger: fix WCG flag in DisplayDevice creation

When a display does not support any wide color mode, yet it is
plugged when mForceNativeColorMode is true, we would incorrectly
mark the display as supporting wide color.

We should ignore mForceNativeColorMode entirely in DisplayDevice
creation because mForceNativeColorMode may change at runtime.

Test: builds
Change-Id: Icb245a2e78954b90d6c5461de8f4ef571858d959