i965/blorp: Refactor to get rid of the get_wm_prog virtual function

i965/blorp: Refactor to get rid of the get_wm_prog virtual function

Instead of having a virtual member function for getting the WM/PS kernel,
we simply add fields for prog_data and the kernel to brw_blorp_parms and
always make sure those get set as part of the different constructors.

v2: Use use prog_data != NULL to check for a valid program instead of a
    magic kernel offset value

Reviewed-by: Topi Pohjolainen <topi.pohjolainen@intel.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>