main/get: Converted type conversion macros to inline functions

main/get: Converted type conversion macros to inline functions

Quiet warnings when called with a GLubyte:

  src/mesa/main/get.c:3215:19: warning: result of comparison of constant 32767 with expression of type 'GLubyte' (aka 'unsigned char') is always false [-Wtautological-constant-out-of-range-compare]
      params[0] = INT_TO_FIXED(((GLubyte *) p)[0]);
                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  src/mesa/main/get.c:78:38: note: expanded from macro 'INT_TO_FIXED'
                                 ~~~ ^ ~~~~~~~~

Delete ENUM_TO_INT64, ENUM_TO_FIXED and BOOLEAN_TO_INT64 which aren't
used.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3866>