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 <fa9c81d52bf7e42f4eca779fafc7ac8cc1c787d8@gmail.com>
Reviewed-by: Michel Dänzer <242b79f0804cf52679d58f27ed754158c1323c75@amd.com>
Reviewed-by: Marek Olšák <74bac9a67a04041c3586cda013cc5a11352e67be@gmail.com>

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