i965: fix blorp stage_prog_data->param leak

i965: fix blorp stage_prog_data->param leak

Patch uses mem_ctx for allocation to ensure param array gets freed
later.

==6164== 48 bytes in 1 blocks are definitely lost in loss record 61 of 193
==6164==    at 0x4C2EB6B: malloc (vg_replace_malloc.c:299)
==6164==    by 0x12E31C6C: ralloc_size (ralloc.c:121)
==6164==    by 0x130189F1: fs_visitor::assign_constant_locations() (brw_fs.cpp:2095)
==6164==    by 0x13022D32: fs_visitor::optimize() (brw_fs.cpp:5715)
==6164==    by 0x13024D5A: fs_visitor::run_fs(bool, bool) (brw_fs.cpp:6229)
==6164==    by 0x1302549A: brw_compile_fs (brw_fs.cpp:6570)
==6164==    by 0x130C4B07: blorp_compile_fs (blorp.c:194)
==6164==    by 0x130D384B: blorp_params_get_clear_kernel (blorp_clear.c:79)
==6164==    by 0x130D3C56: blorp_fast_clear (blorp_clear.c:332)
==6164==    by 0x12EFA439: do_single_blorp_clear (brw_blorp.c:1261)
==6164==    by 0x12EFC4AF: brw_blorp_clear_color (brw_blorp.c:1326)
==6164==    by 0x12EFF72B: brw_clear (brw_clear.c:297)

Fixes: 8d90e28839 ("intel/compiler: Allocate pull_param in assign_constant_locations")
Signed-off-by: Tapani Pälli <tapani.palli@intel.com>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Cc: mesa-stable@lists.freedesktop.org