msm: mdss: hdmi: NUL-terminate spd data

NUL-terminate SPD Vendor name and SPD
Product description data stored in the
control structure of hdmi driver. This
avoids overflows in print messages and info
frames.

Change-Id: I0a0adcfad06625e4d7368d3c5cd0ec38f0e047d8
CRs-Fixed: 520148
Signed-off-by: Manoj Rao <manojraj@codeaurora.org>