mesa/main: Make legal_get_tex_level_parameter_target public

mesa/main: Make legal_get_tex_level_parameter_target public

It will be used by the ARB_internalformat_query2 implementation
to check if the target is valid for those <pnames> that are said
in the spec that should return the same values than the
'glGetTexLevelParameter{if}v' function:

    - INTERNALFORMAT_RED_SIZE
    - INTERNALFORMAT_GREEN_SIZE
    - INTERNALFORMAT_BLUE_SIZE
    - INTERNALFORMAT_ALPHA_SIZE
    - INTERNALFORMAT_DEPTH_SIZE
    - INTERNALFORMAT_STENCIL_SIZE
    - INTERNALFORMAT_SHARED_SIZE
    - INTERNALFORMAT_RED_TYPE
    - INTERNALFORMAT_GREEN_TYPE
    - INTERNALFORMAT_BLUE_TYPE
    - INTERNALFORMAT_ALPHA_TYPE
    - INTERNALFORMAT_DEPTH_TYPE
    - INTERNALFORMAT_STENCIL_TYPE
    - IMAGE_FORMAT_COMPATIBILITY_TYPE

Reviewed-by: Dave Airlie <airlied@redhat.com>