target/mips: use *ctx for DisasContext

target/mips: use *ctx for DisasContext

No changes to the logic here; this is just to make the diff
that follows easier to read.

While at it, remove the unnecessary 'struct' in
'struct TranslationBlock'.

Note that checkpatch complains with a false positive:
  ERROR: space prohibited after that '&' (ctx:WxW)
  #75: FILE: target/mips/translate.c:20220:
  +    ctx->kscrexist = (env->CP0_Config4 >> CP0C4_KScrExist) & 0xff;
                                                              ^
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Cc: Aurelien Jarno <aurelien@aurel32.net>
Cc: Yongbok Kim <yongbok.kim@mips.com>
Signed-off-by: Emilio G. Cota <cota@braap.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>