ARB_clear_texture: fix GL_CLEAR_TEXTURE query check for unsupported formats

It is OK to return GL_NONE even if the specific format supports clearing, but there is no HW support for the format.

Noticed because it currently spams over 2000 ERROR - Piglit error: ... lines in r300 piglit logs: https://gitlab.freedesktop.org/mesa/mesa/-/jobs/66244656