mesa: Generate FRAMEBUFFER_INCOMPLETE_MISSING_ATTACHMENT earlier

mesa: Generate FRAMEBUFFER_INCOMPLETE_MISSING_ATTACHMENT earlier

The ARB_framebuffer_object spec lists this case before the
FRAMEBUFFER_INCOMPLETE_DRAW_BUFFER and
FRAMEBUFFER_INCOMPLETE_READ_BUFFER cases.

Fixes two broken cases in piglit's fbo-incomplete test, if
ARB_ES2_compatibility is not advertised. (If it is, this is masked
because the FRAMEBUFFER_INCOMPLETE_DRAW_BUFFER /
FRAMEBUFFER_INCOMPLETE_READ_BUFFER cases are removed by that extension)

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
Reviewed-by: Brian Paul <brianp@vmware.com>