[AMDGPU] Introduce separate register limit bias in scheduler

Current implementation abuses ErrorMargin to apply an additional
bias to VGPR and SGPR limits under a high register pressure. The
ErrorMargin exists to account for inaccuracies of the RP tracker
and not to tackle an excess pressure. Introduce separate bias for
this purpose and also make it different for SGPRs and VGPRs as we
may want to use different values in the future.

This is supposed to be NFC, however there is a subtle difference
when subtracting a margin overflows the limit. Doing two subtractions
makes it less probable, although manifests only in mir tests with
an artificially small register budget.

Differential Revision: https://reviews.llvm.org/D142051