drm/amd/display: Clean up errors & warnings in amdgpu_dm.c

Fix the following errors & warnings reported by checkpatch:

ERROR: space required before the open brace '{'
ERROR: space required before the open parenthesis '('
ERROR: that open brace { should be on the previous line
ERROR: space prohibited before that ',' (ctx:WxW)
ERROR: else should follow close brace '}'
ERROR: open brace '{' following function definitions go on the next line
ERROR: code indent should use tabs where possible

WARNING: braces {} are not necessary for single statement blocks
WARNING: void function return statements are not generally useful
WARNING: Block comments use * on subsequent lines
WARNING: Block comments use a trailing */ on a separate line

Cc: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Cc: Aurabindo Pillai <aurabindo.pillai@amd.com>
Cc: Alex Deucher <alexander.deucher@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>