Fix LutShader on graphite.

Graphite wants all SkImages to be graphite-backed, which excludes
raster-backed images. Since the LUT was backed by a CPU bitmap, this
borked.

So, explicitly convert the LUT's bitmap into a graphite-backed image.

Bug: 424064905
Flag: EXEMPT bug fix
Test: SilkFX LUT test
Test: CtsSurfaceControlTests
Change-Id: I0195421df2d965cf8014f5a8f573ca0ad3324e19