i965/blorp: Skip uploading state/options not needed for clears

i965/blorp: Skip uploading state/options not needed for clears

In case there is no source it means the program does a simple
clear or a resolve. In such case there is no need to program
sampling state or enable pixel kill in fragment shader.

Signed-off-by: Topi Pohjolainen <topi.pohjolainen@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>