x86: make i915 as a kmod with required firmware

x86: make i915 as a kmod with required firmware

i915 driver requires to load correct firmware to work on latest x86
GPU, it is more reasonable to make it as a kernel module, so that
initramfs is not required, and it can also save some space from the
kernel image comparing being a built-in driver

Signed-off-by: Joe Zheng <joe.zheng@intel.com>
Link: https://github.com/openwrt/openwrt/pull/16276
Signed-off-by: Robert Marko <robimarko@gmail.com>
(cherry picked from commit 77cfe8fd15d3d0f77ee16660d06a174f41b53444)
Link: https://github.com/openwrt/openwrt/pull/17097
Signed-off-by: Petr Štetiar <ynezz@true.cz>