OMAP: DSS2: HDMI: clean up edid reading & fix checksum

Clean up reading of EDID by passing direct address to the block being
read, instead of start address of the whole EDID memory area. Rewrite
the loop which reads the EDID.

This also fixes the checksum calculation, which used to calculate the
checksum only for the first block.

Cc: Mythri P K <mythripk@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>