radeonsi: Fix off-by-one for maximum vertex element index in some cases

radeonsi: Fix off-by-one for maximum vertex element index in some cases

In cases where the vertex element size is smaller than the vertex buffer
stride, the previous calculation could end up 1 too low. This would result
in the GPU using index 0 instead of the maximum index for those elements,
which would be visible as intermittent distorted triangles.

NOTE: This is a candidate for the 9.1 branch.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>