nv50/ir,nvc0: use constant buffers for compute when possible on Kepler+

nv50/ir,nvc0: use constant buffers for compute when possible on Kepler+

Gives a +7.79% increase in FPS with Hitman on lowest quality settings on
my GTX 1060.

total instructions in shared programs : 5787979 -> 5748677 (-0.68%)
total gprs used in shared programs    : 669901 -> 669373 (-0.08%)
total shared used in shared programs  : 548832 -> 548832 (0.00%)
total local used in shared programs   : 21068 -> 21064 (-0.02%)

                local     shared        gpr       inst      bytes
    helped           1           0         152         274         274
      hurt           0           0           0           0           0

Signed-off-by: Rhys Perry <pendingchaos02@gmail.com>
Reviewed-by: Karol Herbst <kherbst@redhat.com>