i965/msaa: Disable unsupported formats.

i965/msaa: Disable unsupported formats.

Due to hardware limitations, MSAA is unsupported on Gen6 for formats
containing >64 bits of data per pixel.  From the Sandy Bridge PRM,
vol4 part1, p72 ("Surface Format"):

    If Number of Multisamples is set to a value other than
    MULTISAMPLECOUNT_1, this field cannot be set to the following
    formats:
    - any format with greater than 64 bits per element
    - any compressed texture format (BC*)
    - any YCRCB* format

Gen7 has a similar, but less stringent limitation: formats with >64
bits of data per pixel only support 4x MSAA.

This patch causes the unsupported formats to report
GL_FRAMEBUFFER_UNSUPPORTED.

Fixes piglit "multisample-formats" tests on Gen6.

Acked-by: Kenneth Graunke <kenneth@whitecape.org>