[graphite] Fix bit generation in TextureFormatTest

Before hand, we were never generating 32-bit UNorm values. After
https://skia-review.git.corp.google.com/c/skia/+/1342676, alpha
channels might be changed to the Pad type, which falls through to
UNorm generation. In the case of a RGBx / unknown alpha applied to
RGBA32F formats, we were now generating Pad/UNorm values for 32 bits
and running into the UB issue of shifting (1 << 32), which SHOULD
JUST BE ZERO, but is not because C.

Change-Id: Id26d79fea829cc8fd1b92e55f8f2b74848d5e137
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1348076
Commit-Queue: Michael Ludwig <michaelludwig@google.com>
Reviewed-by: Thomas Smith <thomsmit@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>