i965: Move up duplicated fields from stage-specific prog_data to brw_stage_prog_data.

i965: Move up duplicated fields from stage-specific prog_data to brw_stage_prog_data.

There doesn't seem to be any reason for nr_params, nr_pull_params,
param, and pull_param to be duplicated in the stage-specific
subclasses of brw_stage_prog_data.  Moving their definition to the
common base class will allow some code sharing in a future commit, the
removal of brw_vec4_prog_data_compare and brw_*_prog_data_free, and
the simplification of the stage-specific brw_*_prog_data_compare.

Reviewed-by: Paul Berry <stereotype441@gmail.com>