mesa: add SYSTEM_VALUE_SAMPLE_MASK_IN

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
glsl: add gl_SampleMaskIn[] builtin

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
i965/fs: add support for gl_SampleMaskIn[]

v2: add assert so we don't run into trouble on Gen6.

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>