mesa: add support for immutable textures to teximagemultisample()

The new entrypoints will come later, but this adds the actual logic for
supporting immutable multisample textures:

- The immutability flag is set as desired.
- Attempting to modify an immutable multisample texture produces
  INVALID_OPERATION.

Note: The extension spec does not mention adding this behavior to
TexImage*Multisample, but it seems like the reasonable thing to do.

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
glapi: add definition of ARB_texture_storage_multisample

Adds XML for the extension, dispatch_sanity enabling, and the two new
entrypoints. These are both implemented by calling the shared
teximagemultisample() with immutable=GL_TRUE.

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
mesa: add enable bit for ARB_texture_storage_multisample
i965: enable ARB_texture_storage_multisample on Gen6+

This can be enabled everywhere that ARB_texture_multisample is
supported -- ARB_texture_storage is supported on everything.

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>