libgrate/fragment_asm: don't clobber rD if it's disabled
libgrate/fragment_asm: don't clobber rD if it's disabled

Tegra114 uses all rD bits even if rD is disabled for conveying additional
opcodes that are specific to Tegra114 GR3D. Hence these bits should be zeroed
if rD is disabled, otherwise Tegra114 will execute the additional opcodes.
This fixes asm_tests on Tegra114.

grate-driver/grate,grate-driver/grate