drm/amd/display: Remove unnecessary casts in amdgpu_dm_helpers.c

drm/amd/display: Remove unnecessary casts in amdgpu_dm_helpers.c

Fixes the following category of checkpatch complaints:

WARNING: unnecessary cast may hide bugs, see http://c-faq.com/malloc/mallocnocast.html
+               char *buf = (char *)kvcalloc(total, sizeof(char), GFP_KERNEL);

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: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>