freedreno/ir3: Use output type size to set OUTPUT_REG_HALF_PRECISION

freedreno/ir3: Use output type size to set OUTPUT_REG_HALF_PRECISION

Previously the A5XX_SP_FS_OUTPUT_REG_HALF_PRECISION was set depending
on whether half_precision was set in the shader key. With support for
mediump precision, it is possible to have different outputs use
different precisions. That means we can’t have a global shader state
to specify it. Instead it now tries to copy the half-float-ness
from the nir_variable for the output into the ir3_shader_variant. This
is then used to decide whether to set half-precision for each output.

The a6xx version is copied from the a5xx code but it has not been
tested.

v2. [Hyunjun Ko (zzoon@igalia.com)] There's the half flag recently
added, which represents precision based on IR3_REG_HALF. Now use this
flag to avoid duplication.

Signed-off-by: Rob Clark <robdclark@chromium.org>