CCTexture2D.cpp: Respect _PVRHaveAlphaPremultipled flag in initWithImage()

Currently, the ```_PVRHaveAlphaPremultipled``` is ignored, because the code path that PVR loading takes never reaches the required check. 

When a PVR texture is being loaded, ```Image``` gets passed to ```initWithData``` and then ```initWithMipmaps``` (which forces the ```_hasPremultipliedAlpha``` to false).
    
This patch moves the check before any init methods so that the ```_hasPremultipledAlpha``` flag is set correctly for all texture types. For non-PVR it defaults to false if unknown (same as before).