ir3: Unconditionally enable MERGEDREGS on a6xx

ir3: Unconditionally enable MERGEDREGS on a6xx

As per discussion on !5059, we don't see any particular reason as to
why MERGEDREGS should be disabled on HS/DS/GS, and none of the dEQP
tests (both VK and GL) fail when MERGEDREGS is enabled. In fact, some
of the VK dEQP tests fail when MERGEDREGS is disabled (e.g. tests
with shaders that employ a0.x). As a result, let's just enable
MERGEDREGS unconditionally on a6xx.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5059>