u_blit,u_simple_shaders: add shader to convert from xrbias format

u_blit,u_simple_shaders: add shader to convert from xrbias format

We need this to handle some oddball dx10 format
(DXGI_FORMAT_R10G10B10_XR_BIAS_A2_UNORM). What you can do with this
format is very limited, hence we don't want to add it as a gallium
format (we could not express the properties of this format as
ordinary format properties neither, so like all special formats
it would need specific code for handling it in any case).
While here, also nuke the array for different shaders for different
writemasks, as it was not actually used (always full masks are
passed in for generating shaders).

Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Jose Fonseca <jfonseca@vmware.com>