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: e5604ef78b "st/mesa/i965: Allow decompressing ETC2 to GL_RGBA"

Reviewed-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Reviewed-by: Adam Jackson <ajax@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/8529>

src/mesa/main/texcompress_etc.c

@@ -750,8 +750,8 @@ etc2_unpack_srgb8(uint8_t *dst_row,
 		  tmp = dst[0];
 		  dst[0] = dst[2];
 		  dst[2] = tmp;
-		  dst[3] = 255;
 	       }
+               dst[3] = 255;
 
                dst += comps;
             }