Default to RGB for Bitmap Buffers in svp.

By default we seem to use FORMAT_TWENTYFOUR_BIT_TC_MASK for most of our
buffers, and presumably we should be using RGB as that's the default
format expected by anyone else -- previously red and blue channels
were switched when e.g. using tiled rendering on iOS (the vcl implementation
is built on svp and appears to therefore reuse svpbmp without change), on
Android (which uses svp directly) and in experimental tiled rendering
on Linux with svp.

Change-Id: I66a7f83bce286b7b2fddf0010587b019e8e1d01f