arb_blend_func_extended: gles: follow the specification requirements

arb_blend_func_extended: gles: follow the specification requirements

As a side-effect of the simplification, the test now will be skipped if
the MAX_DRAW_BUFFER requirements (8 respectively 4 for ES) are not met.

GL_MAX_DRAW_BUFFERS
    params returns one value ... The value must be at least 4.
Ref: https://registry.khronos.org/OpenGL-Refpages/es3.0/html/glGet.xhtml

Currently relevant for Adreno 300 series and Raspberry Pi 4 which both
meet OpenGL ES requirement.

Fixes: b6d3ff45f476 ("Implement piglit tests for EXT_blend_func_extended")
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Signed-off-by: David Heidelberg <david.heidelberg@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/piglit/-/merge_requests/916>