assembler: Don't warn if identical declared registers are redefined

assembler: Don't warn if identical declared registers are redefined

There's no real need to warn when the same register is declared twice.
Currently the libva driver does do that and this warning makes other
errors really hide in a sea of warnings.

Redefining a register with different parameters is a real error though,
so we should not allow that and error out in that case.

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>