drm/amd/display: Fix unused variable warning

[Why]
The dm struct is only being used if DCN config is defined and this
causes a unused variable warning if DCN option is not set.

[How]
Remove the compile flag so the variable is used (there also seems to be
a duplicate guard due to a bad rebase) so remove the outer guard to fix
the warning.

Fixes this warning
drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c: In function 'dm_set_vblank':
drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c:5380:33: warning: unused variable 'dm' [-Wunused-variable]

Fixes: 98ab5f35 ("drm/amd/display: Fix deadlock during gpu reset v3")
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>