drm/amd/display: Fix issue with VLine interrupt not firing

[Why]
We are not correctly handling the wrap around case.
VLine interrupt is relative to position of VUpdate interrupt.

Both VUpdate interrupt and VLine interrupt could possibly
be in front porch or back porch.

[How]
Fix wraparound case by checking for line number that is
greater than the VTOTAL of the OTG timing. In this case,
the interrupt should occur on the next frame.

Also fix some variable naming and remove some dead code.

Signed-off-by: Krunoslav Kovac <Krunoslav.Kovac@amd.com>
Reviewed-by: Anthony Koo <Anthony.Koo@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Acked-by: Tony Cheng <Tony.Cheng@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>