r600g: Fix out of bounds access

r600g: Fix out of bounds access

fc_sp variable should indicate number of elements in
fc_stack array, but fc_sp was increased at beginning of fc_pushlevel
function. It leads to situation where idx=0 was never used, and last
32 element was stored outside fs_stack array.

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