r600g: Use R600_MAX_VIEWPORTS instead of 16

r600g: Use R600_MAX_VIEWPORTS instead of 16

Lets define R600_MAX_VIEWPORTS instead of using 16 here and there
in the code when looping through viewports and scissors. It is
easier to understand what this number represents.

v2: Missed a case where R600_MAX_VIEWPORTS should have been used.

Signed-off-by: Alexandre Demers <alexandre.f.demers@gmail.com>
Signed-off-by: Marek Olšák <marek.olsak@amd.com>