automake: r600: radeonsi: correctly manage libamd_common.la linking

automake: r600: radeonsi: correctly manage libamd_common.la linking

Since both r600 and radeonsi use code from libamd_common they need to
static link it. At the same time, adding a common library to LIB_DEPS is
fragile [can lean to multiple symbol definitions] and non-obvious - I
had to do a double-take how things work atm.

So follow the libradeon.la approach and put common libraries in
TARGET_RADEON_COMMON

Fixes: 936f5407a7d ("gallium/radeon: Add libamd_common.a to TARGET_LIB_DEPS also for r600")
Cc: Timothy Arceri <tarceri@itsqueeze.com>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Acked-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Tested-by: Michel Dänzer <michel.daenzer@amd.com>