drm/msm/dp: fix reading of edid extension blocks

EDID extension block reads are failing due to the reset of
offset variable at incorrect state. This change removes the
offset variable reset to use the correct offset value in
reading the EDID extension blocks.

CRs-Fixed: 2256731
Change-Id: I90b599520aa0562e12157193505181ea2573bca1
Signed-off-by: Govinda Rajulu Chenna <gchenna@codeaurora.org>