vline packet parsing function for R600 and Evergreen+ are
the same, except that they use different registers. Factor
out the algorithm into a common function that uses register
table passed from ASIC-specific caller.

This reduces ASIC-specific function to (trivial) setup
of register table and call into the common function.
Signed-off-by: NIlija Hadzic <ihadzic@research.bell-labs.com>
Reviewed-by: NMarek Olšák <maraeo@gmail.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>