drm/amdgpu/display: Remove unnecessary typecasts and fix build issues

[Why]
Getting below errors:
drivers/gpu/drm/amd/amdgpu/../display/dc/dml/dcn32/display_mode_vba_32.c:1414:5: error: implicit conversion from enumeration type 'enum scan_direction_class' to different enumeration type 'enum dm_rotation_angle' [-Werror,-Wenum-conversion]
                                mode_lib->vba.SourceScan[k],
                                ^~~~~~~~~~~~~~~~~~~~~~~~~~~
drivers/gpu/drm/amd/amdgpu/../display/dc/dml/dcn32/display_mode_vba_32.c:1744:22: error: implicit conversion from enumeration type 'enum scan_direction_class' to different enumeration type 'enum dm_rotation_angle' [-Werror,-Wenum-conversion]
                        && (!(!IsVertical(mode_lib->vba.SourceScan[k])) || mode_lib->vba.DCCEnable[k] == true)) {
                               ~~~~~~~~~~ ^~~~~~~~~~~~~~~~~~~~~~~~~~~
2 errors generated.

drivers/gpu/drm/amd/amdgpu/../display/dc/dml/dcn32/display_mode_vba_util_32.c:5484:18: error: implicit conversion from enumeration type 'RequestType' to different enumeration type 'enum RequestType' [-Werror,-Wenum-conversion]
                        RequestLuma = REQ_256Bytes;
                                    ~ ^~~~~~~~~~~~
18 errors of similar kind

[How]
1. Add typecast at relevant places
2. Move the enum RequestType definition ahead of declarations

Signed-off-by: Chandan Vurdigere Nataraj <chandan.vurdigerenataraj@amd.com>
Reviewed-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>