drm/amd/display: Fix for NULL ramp pointer crashing driver

[Why]
In certain scenarios the ramp parameter come in as NULL, which crashes
because this function doesn't guard properly in the early return.

[How]
- parameter mapUserRamp should be the guard (false means no ramp)
- remove checking ramp in early return

Signed-off-by: Joshua Aberback <joshua.aberback@amd.com>
Reviewed-by: Sivapiriyan Kumarasamy <Sivapiriyan.Kumarasamy@amd.com>
Acked-by: Eryk Brol <Eryk.Brol@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>