Technically it won't work without, but it's possible to build the firmware into the kernel, so use RRECOMMENDS.
(From meta-openembedded rev:
fb6ed3dfe6596aa9f511563d0c7274cac4274236)
Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Patrick Ohly <patrick.ohly@intel.com>
DEPENDS += "virtual/libx11 libxvmc drm xf86driproto glproto \
virtual/libgl xineramaproto libpciaccess"
RDEPENDS_${PN} += "xserver-xorg-module-exa"
+RRECOMMENDS_${PN} += "linux-firmware"
COMPATIBLE_HOST = '(i.86|x86_64).*-linux'