r600g,radeonsi: Fix calculation of IR target cap string buffer size

r600g,radeonsi: Fix calculation of IR target cap string buffer size

Fixes writing beyond the allocated buffer:

==31855== Invalid write of size 1
==31855==    at 0x50AB2A9: vsprintf (iovsprintf.c:43)
==31855==    by 0x508F6F6: sprintf (sprintf.c:32)
==31855==    by 0xB59C7EC: r600_get_compute_param (r600_pipe_common.c:526)
==31855==    by 0x5B2B7DE: get_compute_param<char> (device.cpp:37)
==31855==    by 0x5B2B7DE: clover::device::ir_target() const (device.cpp:201)
==31855==    by 0x5B398E0: clover::program::build(clover::ref_vector<clover::device> const&, char const*, clover::compat::vector<clover::compat::pair<clover::compat::string, clover::compat::string> > const&) (program.cpp:63)
==31855==    by 0x5B20152: clBuildProgram (program.cpp:182)
==31855==    by 0x400F41: main (hello_world.c:109)
==31855==  Address 0x56fed5f is 0 bytes after a block of size 15 alloc'd
==31855==    at 0x4C29180: operator new(unsigned long) (vg_replace_malloc.c:324)
==31855==    by 0x5B2B7C2: allocate (new_allocator.h:104)
==31855==    by 0x5B2B7C2: allocate (alloc_traits.h:357)
==31855==    by 0x5B2B7C2: _M_allocate (stl_vector.h:170)
==31855==    by 0x5B2B7C2: _M_create_storage (stl_vector.h:185)
==31855==    by 0x5B2B7C2: _Vector_base (stl_vector.h:136)
==31855==    by 0x5B2B7C2: vector (stl_vector.h:278)
==31855==    by 0x5B2B7C2: get_compute_param<char> (device.cpp:35)
==31855==    by 0x5B2B7C2: clover::device::ir_target() const (device.cpp:201)
==31855==    by 0x5B398E0: clover::program::build(clover::ref_vector<clover::device> const&, char const*, clover::compat::vector<clover::compat::pair<clover::compat::string, clover::compat::string> > const&) (program.cpp:63)
==31855==    by 0x5B20152: clBuildProgram (program.cpp:182)
==31855==    by 0x400F41: main (hello_world.c:109)

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Tom Stellard <thomas.stellard@amd.com>