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>