i965: Use util_next_power_of_two() for brw_get_scratch_size()

i965: Use util_next_power_of_two() for brw_get_scratch_size()

This function computes the next power of two, but at least 1024. We can
do that by bitwise or'ing in 1023 and calling util_next_power_of_two().

We use brw_get_scratch_size() from the compiler so we need it out of
brw_program.c. We could move it to brw_shader.cpp, but let's make it a
small inline function instead.

Reviewed-by: Topi Pohjolainen <topi.pohjolainen@intel.com>
Signed-off-by: Kristian Høgsberg Kristensen <krh@bitplanet.net>