mesa: new _mesa_prepare_mipmap_level() function for mipmap generation

mesa: new _mesa_prepare_mipmap_level() function for mipmap generation

This helper function is used during mipmap generation to prepare space
for the destination mipmap levels.

This improves/fixes two things:
1. If the texture object was created with glTexStorage2D, calling
   _mesa_TexImage2D() to allocate the new image would generate
   INVALID_OPERATION since the texture is marked as immutable.
2. _mesa_TexImage2D() always frees any existing texture image memory
   before allocating new memory.  That's inefficient if the existing
   image is the right size already.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Eric Anholt <eric@anholt.net>