gallium: add PIPE_CAP_TGSI_CAN_READ_OUTPUTS

gallium: add PIPE_CAP_TGSI_CAN_READ_OUTPUTS

Drivers that support this benefit by saving one lowering pass in the
GLSL-to-TGSI conversion.

radeonsi already supports this because all outputs are stored in temporary
variables before the export (except for TCS outputs, which have always
been readable in TGSI anyway due to their special semantics).

Reviewed-by: Marek Olšák <marek.olsak@amd.com>