graphics/mesa-{libs,dri}: update to 18.2.8

• TEXTURE option is always enabled per https://cgit.freedesktop.org/mesa/mesa/commit/?id=66673bef941a

Changes: https://www.mesa3d.org/relnotes/18.2.0.html
Changes: https://www.mesa3d.org/relnotes/18.2.1.html
Changes: https://www.mesa3d.org/relnotes/18.2.2.html
Changes: https://www.mesa3d.org/relnotes/18.2.3.html
Changes: https://www.mesa3d.org/relnotes/18.2.4.html
Changes: https://www.mesa3d.org/relnotes/18.2.5.html
Changes: https://www.mesa3d.org/relnotes/18.2.6.html
Changes: https://www.mesa3d.org/relnotes/18.2.7.html
Changes: https://www.mesa3d.org/relnotes/18.2.8.html
PR: 230298
Tested by: Samy Mahmoudi, Kevin Oberman
Approved by: maintainer timeout (2 weeks after 2019Q1)
Differential Revision: https://reviews.freebsd.org/D16571