Fix a number of texture compression issues.

Fix a number of texture compression issues.
Pass the MESA_FORMAT_* token to the _mesa_compressed_row_stride(),
_mesa_compressed_texture_size() and _mesa_compressed_image_address()
functions since we want to use the driver-chosen format, not the user's
internalFormat hint.
Consolidate code related to choosing the texture format in texstoree.c