package/mesa3d: fix iris driver depends and selects

package/mesa3d: fix iris driver depends and selects

iris is inherently an x86-only driver, and it hard codes gcc options
specific to x86m like -msse2, causing build breakage on other
architectures.

iris also does not use kmsro, but the select was accidentally added when
iris was introduced.

Fix both by adding the missing dependency to x86, and by removing the
select to kmsro.

Signed-off-by: James Hilliard <james.hilliard1@gmail.com>
[yann.morin.1998@free.fr:
  - ad dependency to x86
  - reword commit log accordingly
]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>