i965: Don't check for format differences when using the blorp blitter

i965: Don't check for format differences when using the blorp blitter

Previously the blorp blitter wouldn't be used if the source and destination
buffer had a different format other than swizzling between RGB and BGR and
adding or removing a dummy alpha channel. However there's no reason why the
blorp code path can't be used to do almost all format conversions so this
patch just removes the checks. However it does explicitly disable converting
to/from MESA_FORMAT_Z24_UNORM_X8_UINT because there is a similar check
brw_blorp_copytexsubimage.

This doesn't cause any Piglit test regressions at least on Ivybridge.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>