i965/vec4: Use NIR remapping for VS attributes

i965/vec4: Use NIR remapping for VS attributes

The NIR pass already handles remapping system values to attributes for
us so we delete the system value code as part of the conversion.

We also change nir_lower_vs_inputs to take an explicit inputs_read
bitmask and pass in the inputs_read from prog_data instead from pulling
it out of NIR.  This is because the version in prog_data may get
EDGEFLAG added to it on some old platforms.

Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>