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: b6d3ff45 ("Implement piglit tests for EXT_blend_func_extended") Signed-off-by: David Heidelberg david.heidelberg@collabora.com