[Mesa-dev] Fix regression after radeon GenerateMipmap changes

[Will Dyson]

My change to image_matches_texture_obj() caused a regression when using
texImage with level > MaxLevel and pixels != null. Revert it, as the orignal
check was correct. Instead, change _mesa_meta_GenerateMipmap() to temporarily
raise MaxLevel when calling texImage. This allows radeon to correctly allocate
miptree space for the new level.