drm/msm/dp: validate inputs for simulated AUX transfers

Validate the inputs for simulated AUX transfers to ensure that
access to the test data for DPCD and EDID is not out of the
bounds of what is allocated. Furthermore, reset the offset for
AUX transfers that are not related to reading EDID data.

Change-Id: I88fba6e2bd9861e3a6489c22d2e91e78eafbc456
Signed-off-by: Tatenda Chipeperekwa <tatendac@codeaurora.org>