msm_fb: hdmi: Correct checksum value

This change writes the checksum value to registers after
doing all the calculations so that the correct checksum
value is transmitted.

CRs-Fixed: 388272
Change-Id: Ia0543e458d5a11b148bd29e0d57ae355a051b2a9
Signed-off-by: Ajay Singh Parmar <aparmar@codeaurora.org>