vdpau_trace: Fix GCC 4.8 build warnings

vdpau_trace.cpp: In function 'void _vdp_cap_dump_video_mixer_attribute_value(VdpVideoMixerAttribute, const void*, bool)':
vdpau_trace.cpp:539:48: error: cast from type 'const void*' to type 'const float (**)[3][4]' casts away qualifiers [-Werror=cast-qual]
                 ptr = *(VdpCSCMatrix const * *)value;
                                                ^
vdpau_trace.cpp: In function 'void _vdp_cap_dump_bitstream_buffer_list(uint32_t, const VdpBitstreamBuffer*)':
vdpau_trace.cpp:1175:52: error: cast from type 'const void* const' to type 'uint8_t* {aka unsigned char*}' casts away qualifiers [-Werror=cast-qual]
             uint8_t * ptr = (uint8_t * )buffers[0].bitstream;
                                                    ^
Signed-off-by: Robert Morell <rmorell@nvidia.com>
Reviewed-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Aaron Plattner <aplattner@nvidia.com>

trace/vdpau_trace.cpp

@@ -536,7 +536,7 @@ static void _vdp_cap_dump_video_mixer_attribute_value(
             // the value, so it can either fill in the value, or NULL out the
             // pointer.
             if (get_operation) {
-                ptr = *(VdpCSCMatrix const * *)value;
+                ptr = *(VdpCSCMatrix const * const *)value;
             }
             else {
                 ptr = (VdpCSCMatrix const *)value;
@@ -1172,7 +1172,7 @@ static void _vdp_cap_dump_bitstream_buffer_list(
             buffers[0].bitstream_bytes
         );
         if (_vdp_cap_data.level >= LEVEL_DATA) {
-            uint8_t * ptr = (uint8_t * )buffers[0].bitstream;
+            const uint8_t * ptr = (const uint8_t * )buffers[0].bitstream;
             for (uint32_t i = 0; i < buffers[0].bitstream_bytes; ++i) {
                 fprintf(_vdp_cap_data.fp, "%02x ", ptr[i]);
             }