[graphite] Remove ReadSwizzle enum

This enum was represented an overly restricted point of brittleness,
and had been needed when we applied the swizzle by branching in the
shader. Now that the swizzle is applied via a few parameters for alpha
and with the gamut column reordering, we can derive the same values
from the original skgpu::Swizzle object with fewer asserts.

I also removed extra swizzle concatenations for alpha-only color
types, since the swizzle should already be doing that if it needs to.

Lastly, found a bug in the dst-OOTF values, which was also swizzling
its RGB components like the src-OOTF values. These are applied on
either side of the gamut matrix transform, so only the src needs to
be swizzled.

Bug: b/390473370
Change-Id: Ia146403765f178c737d25b216d015e3edd2fe417
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1166337
Reviewed-by: Nicolette Prevost <nicolettep@google.com>
Commit-Queue: Michael Ludwig <michaelludwig@google.com>