usb_pd: Redo the change of DP alt-mode event
usb_pd: Redo the change of DP alt-mode event

The CL:1685787 changed the DP alt-mode event from the host event
MODE_CHANGE to the MKBP event EC_MKBP_EVENT_DP_ALT_MODE_ENTERED.

Later a refactoring CL:2132871 merged the logic of TCPMv1 and
TCPMv2 stacks and accidentally changed it back to MODE_CHANGE. As
TCPMv2 still used the old event.

This CL redoes the first CL to use the MKBP event
EC_MKBP_EVENT_DP_ALT_MODE_ENTERED.

BRANCH=None
BUG=chromium:786721, b:170893877
TEST=Triggered HPD L->H event in S3 and checked the new MKBP event
is sent.

Change-Id: I5a8f35d40ad8e724841c771738c01a381e163250
Signed-off-by: Wai-Hong Tam <04b587fdf5845741a0c5a8b9cd59ca72d73ef8fc@google.com>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/2504476
Reviewed-by: Aseda Aboagye <12c9b286316a940fd31c24070da5ab64cdd4d0e7@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec