mesa: add support for immutable textures to teximagemultisample()

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.

V2: - Cover missing error cases (unsized formats; texture object zero)

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
[V1] Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Brian Paul <brianp@vmware.com>