mesa: enable gallium intel drivers when building for x86

Iris is the default, recommended option for Skylake and newer nowadays,
i915 gallium is favoured over classic i915, and soon classic i965 will
be replaced by crocus gallium driver for i965-to-Haswell GPUs.

(From OE-Core rev: 670878b5919c2a67a2fea0b8e1f032e324fad5aa)

Signed-off-by: Alexander Kanavin <alex.kanavin@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>