gallium: replace pipe_type enum with tgsi_return_type enum

gallium: replace pipe_type enum with tgsi_return_type enum

The only place the enum pipe_type was used is for the TGSI sampler
view return type.  So make it a TGSI type.  Note: it appears this
part of TGSI isn't used by anyone so it may be removed in the future.

v2: the new name is tgsi_return_type, not tgsi_type.  This means we
can drop the previously posted tgsi_type -> tgsi_opcode_type patch.

Reviewed-by: Roland Scheidegger <sroland@vmware.com>