meson: Fix with_gallium_icd to with_opencl_icd

`with_gallium_icd` is never used throughout the different Meson build
files, whereas `with_opencl_icd` tracks whether or not `gallium-opencl`
was set to "icd".

Fixes: 42ea0631
         ("meson: build clover")
Signed-off-by: Pierre Moreau <pierre.morrow@free.fr>
Reviewed-by: Dylan Baker <dylan@pnwbakers.com>
(cherry picked from commit 0b736f7f)