gallium/u_blitter: Remove the overlapped blit assert from util_blitter_blit_generic().

gallium/u_blitter: Remove the overlapped blit assert from util_blitter_blit_generic().

This is used by st_BlitFramebuffer() / r600_blit(), and ARB_fbo allows
overlapped blits, even though the result is undefined. No piglit regressions
on r600g / CYPRESS.

Signed-off-by: Henri Verbeet <hverbeet@gmail.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Reviewed-by: Marek Olšák <maraeo@gmail.com>