drm/amd/display: Remove else after return in 'dm_vblank_get_counter' &...

drm/amd/display: Remove else after return in 'dm_vblank_get_counter' & 'amdgpu_dm_backlight_get_level'

Expressions under 'else' branch in function 'dm_vblank_get_counter' &
'amdgpu_dm_backlight_get_level' are executed whenever the expression in
'if' is False. Otherwise, return from function occurs. Therefore, there is
no need in 'else', and it has been removed.

Fixes the following:

WARNING: else is not generally useful after a break or return
+               return 0;
+       else {

WARNING: else is not generally useful after a break or return
+               return convert_brightness_to_user(&caps, avg);
+       } else {

Cc: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Cc: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Srinivasan Shanmugam <srinivasan.shanmugam@amd.com>
Reviewed-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>