lodbias: Reduce the number of glReadPixels calls
Read the framebuffer once per probe_cell call instead of doing so 16 times. With debug builds of piglit (GBM platform) and iris, this change improves the runtime of the test on a TGL by about 12x and allows it to run in the CI without timing out.
The CI timeout issue was reported by @craftyguy.
Edited by Nanley Chery