mesa: Refactor common validation code to validate_DrawElements_common

mesa: Refactor common validation code to validate_DrawElements_common

Most of the code in _mesa_validate_DrawElements,
_mesa_validate_DrawRangeElements, and
_mesa_validate_DrawElementsInstanced was the same.  Refactor this out to
common code.

As a side-effect, a bug in _mesa_validate_DrawElementsInstanced was
fixed.  Previously this function would not generate an error when
check_valid_to_render failed if numInstances was 0.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>