Corrected DdsImagePlugin setting info gamma

[radarhere]

Resolves #4164

The issue reports that the following line fails -

Pillow/src/PIL/DdsImagePlugin.py

Line 158 in 84fed4d

self.im_info["gamma"] = 1 / 2.2

This is understandable, since there is no self.im_info. Presumably, self.info was meant when this was added in #2068.

Setting gamma in info in this way is already done in PNG -

>>> from PIL import Image
>>> im = Image.open("Tests/images/hopper.png")
>>> im.info
{'gamma': 0.45455, 'chromaticity': (0.3127, 0.329, 0.64, 0.33, 0.3, 0.6, 0.15, 0.06), 'dpi': (96, 96)}