mesa: fix missing llvm dependency for i915

mesa: fix missing llvm dependency for i915

i915 requires llvm at least explicitly since commit b8b38d38b1c1
("meson: reinstate LLVM requirement for r300 and enforce it for i915
too") in mesa (available since 25.3.0).

By default, Intel machines are built with gallium-llvm (see the various
PACKAGECONFIG:append: in mesa.bb), so the dependencies would be met.

It seems like Intel Arc discrete GPU may be supported by i915, so I'm
assuming it could be possible to have a non-Intel machine have the need
for the i915 driver.

In any case, this is highlighting the actual dependencies so let's add
gallium-llvm as a dependency for i915, as it should.

Signed-off-by: Quentin Schulz <quentin.schulz@cherry.de>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>