drm/vkms: Avoid reading beyond LUT array

In the Linux kernel, the following vulnerability has been resolved: drm/vkms: Avoid reading beyond LUT array When the floor LUT index (drm_fixp2int(lut_index) is the last index of the array the ceil LUT index will point to an entry beyond the array. Make sure we guard against it and use the value of the floor LUT index. v3: - Drop bits from commit description that didn't contribute anything of value