i965/fs_surface_builder: Only apply predicate to components that exist

i965/fs_surface_builder: Only apply predicate to components that exist

In certain conditions, we have to do bounds-checking in the shader for
image_load_store.  The way this works for image loads is that we do a
predicated load and then emit a series of selects, one per component,
that gives us 0 or the loaded value depending on whether or not you're
in bounds.  However, we were hard-coding 4 components which may not be
correct.  Instead, we should be using size which is the number of
components read.

Reviewed-by: Francisco Jerez <currojerez@riseup.net>