i965/blorp: Add support for source swizzle

i965/blorp: Add support for source swizzle

In order to support cases where gen9 uses RGBA format to back client
requested RGB, one needs to have means to force alpha channel to one
when user requested RGB surface is used as blit source.

v2 (Ken): Use helper for constructing the swizzle (this should be
          changed to use brw_get_texture_swizzle() as a follow-up).
          Also calculate the swizzle for CopyTexSubImage.

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