i965/miptree: Refactor CCS_E and CCS_D cases in render_aux_usage

This commit unifies the CCS_E and CCS_D cases.  This should fix a couple
of subtle issues.  One is that when you use INTEL_DEBUG=norbc to disable
CCS_E, we don't get the sRGB blending workaround.  By unifying the code,
we give CCS_D that workaround as well.

The second issue fixed by this refactor is that the blending workaround
was appears to be enabled on all gens but really only applies on gen9.
Due to a happy accident in the way code was laid out, it was only
getting enabled on gen9: gen8 and earlier don't support non-zero-one
clear colors, and gen10 supports sRGB for CCS_E so it got caught in the
format_ccs_e_compat_with_miptree case.  This refactor moves it above the
format_ccs_e_compat_with_miptree case so it's an explicit early exit and
makes it explicitly only on gen9.

Reviewed-by: Nanley Chery <nanley.g.chery@intel.com>
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Cc: "17.3" <mesa-stable@lists.freedesktop.org>
(cherry picked from commit 361e1df1)