radeonsi/nir: set input_usage_mask properly

radeonsi/nir: set input_usage_mask properly

shader-db results for VEGA64:

Totals from affected shaders:
SGPRS: 791528 -> 792616 (0.14 %)
VGPRS: 421624 -> 410784 (-2.57 %)
Spilled SGPRs: 1639 -> 1674 (2.14 %)
Spilled VGPRs: 0 -> 0 (0.00 %)
Private memory VGPRs: 0 -> 0 (0.00 %)
Scratch size: 0 -> 0 (0.00 %) dwords per thread
Code Size: 16103516 -> 16063696 (-0.25 %) bytes
LDS: 0 -> 0 (0.00 %) blocks
Max Waves: 136307 -> 137830 (1.12 %)
Wait states: 0 -> 0 (0.00 %)

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