radeonsi/gfx9: proper workaround for LS/HS VGPR initialization bug

radeonsi/gfx9: proper workaround for LS/HS VGPR initialization bug

When the HS wave is empty, the hardware writes the LS VGPRs starting at
v0 instead of v2. Workaround by shifting them back into place when
necessary. For simplicity, this is always done in the LS prolog.

According to the hardware team, this will be fixed in future chips,
so take that into account already.

Note that this is not a bug fix, as the bug was already worked
around by commit 166823bfd26 ("radeonsi/gfx9: add a temporary workaround
for a tessellation driver bug"). This change merely replaces the
workaround by one that should be better.

v2: add workaround code to shader only when necessary
v3: clarify the prefer_mono comment

Reviewed-by: Marek Olšák <marek.olsak@amd.com>