mesa: fix alpha channel of ETC2_SRGB8 decompression for !bgra
If software decompression is used for ETC2, the alpha channel for sRGB8 textures would be set only if BGRA is true.
Fixes: e5604ef7 "st/mesa/i965: Allow decompressing ETC2 to GL_RGBA"
If software decompression is used for ETC2, the alpha channel for sRGB8 textures would be set only if BGRA is true.
Fixes: e5604ef7 "st/mesa/i965: Allow decompressing ETC2 to GL_RGBA"