mesa: Fix error generation for glClearBuffer{i ui}v with GL_DEPTH or GL_STENCIL

mesa: Fix error generation for glClearBuffer{i ui}v with GL_DEPTH or GL_STENCIL

The spec says "Only ClearBufferiv should be used to clear
stencil buffers." and "Only ClearBufferfv should be used to clear
depth buffers."  However, on the following page it also says:

    "The result of ClearBuffer is undefined if no conversion between
    the type of the specified value and the type of the buffer being
    cleared is defined (for example, if ClearBufferiv is called for a
    fixed- or floating-point buffer, or if ClearBufferfv is called
    for a signed or unsigned integer buffer). *This is not an error.*"

Emphasis mine.

Fixes problems with piglit's clearbuffer-invalid-drawbuffer test.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Eric Anholt <eric@anholt.net>