mesa: Fix several issues reported by -Wsometimes-uninitialized and -Wenum-conversion

What does this MR do and why?

• frontends/va: Remove wrong use of ProfileToPipe

  The context->templat.profile variable is already of enum pipe_video_profile.

• mesa: Use a switch for state_iter and be more precise about its type

  Even if this technically won't change anything, it prevents a false-positive of uninitialized use of variables with clang.

• gallivm/lp_bld_nir_aos: Use TGSI instead of PIPE enum

  Use the value from the correct enumeration.

• gallivm: Initialize indir_index to NULL before use

  In the case if (var) is false, the value will still be used but uninitialized.

• mesa/ffvs: Use gl_state_index16 in helpers directly