freedreno/ir3: refactor out helpers for comparing shader keys

freedreno/ir3: refactor out helpers for comparing shader keys

Each of the ir3 users has *basically* the same logic for comparing the
previous and current shader key, to see which, if any, shader state
needs to be marked dirty due to shader variant change.

The difference between gen's was just that some lowering flags never get
set on certain generations.  But it doesn't really hurt to include the
extra checks (because both keys would have false).

Signed-off-by: Rob Clark <robdclark@gmail.com>