crocus: add support for set alpha to one with blt.
This is ported from 965 and fixes dEQP-GLES2.functional.fbo.render.recreate_colorbuffer.*rebind_tex2d_rgb* Fixes: f3630548 ("crocus: initial gallium driver for Intel gfx 4-7") Part-of: <mesa/mesa!12164> (cherry picked from commit 842b8c89)