drm/amdkfd: Move gfx12 trap handler to separate file

gfx12 derivatives will have substantially different trap handler
implementations from gfx10/gfx11. Add a separate source file for
gfx12+ and remove unneeded conditional code.

No functional change.

v2: Revert copyright date to 2018, minor comment fixes

Signed-off-by: Jay Cornwall <jay.cornwall@amd.com>
Reviewed-by: Lancelot Six <lancelot.six@amd.com>
Cc: Jonathan Kim <jonathan.kim@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>