[mlir][linalg][transform][python] Simplify mix-in of PadOp.

This patch removes some manual conversion of mixed Python/attribute
arguments to `I64ArrayAttr`s, which turned out to be unnecessary.
Interestingly, this change does not depend on the additional attribute
builders added in the (currently pending)
https://reviews.llvm.org/D159403 patch.

Reviewed By: ftynse

Differential Revision: https://reviews.llvm.org/D159419