drm/i915/firmware: Correct URL for firmware

Replace 01.org URL with upstream linux-firmware repo URL.
We no longer release firmware to 01.org.
linux-firmware.git is the ultimate place to find
the i915 firmwares.

Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Anusha Srivatsa <anusha.srivatsa@intel.com>
Acked-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1525129168-529-1-git-send-email-anusha.srivatsa@intel.com