targets/(r300|r600|radeonsi)/dri: Convert to static/shared pipe-drivers

targets/(r300|r600|radeonsi)/dri: Convert to static/shared pipe-drivers

Related to previous commit, merge the separate dri targets to a single
one.

This is essentially all the buildsystem mayhem required for megaradeon.

Cc: Marek Olšák <marek.olsak@amd.com>
Cc: Michel Dänzer <michel.daenzer@amd.com>
Cc: Christian König <christian.koenig@amd.com>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Tested-by: Rob Clark <robclark@freedesktop.org>
Tested-by: Thomas Helland <thomashelland90 at gmail.com>
Acked-by: Tom Stellard <thomas.stellard@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>