drm/amd/display: Removed redundant @ symbol to fix kernel-doc warnings in -next repo

For linux-next repository.

./drivers/gpu/drm/amd/display/dc/inc/hw/hubp.h:1: warning: no structured comments found
./drivers/gpu/drm/amd/display/dc/inc/hw/mpc.h:132: warning: Incorrect use of kernel-doc format:          * @@overlap_only: Whether overlapping of different planes is allowed.
./drivers/gpu/drm/amd/display/dc/inc/hw/mpc.h:132: warning: Incorrect use of kernel-doc format:          * @@overlap_only: Whether overlapping of different planes is allowed.
./drivers/gpu/drm/amd/display/dc/inc/hw/mpc.h:1: warning: no structured comments found
./drivers/gpu/drm/amd/display/dc/inc/hw/mpc.h:132: warning: Incorrect use of kernel-doc format:          * @@overlap_only: Whether overlapping of different planes is allowed.
./drivers/gpu/drm/amd/display/dc/inc/hw/mpc.h:162: warning: Function parameter or struct member 'pre_multiplied_alpha' not described in 'mpcc_blnd_cfg'

Signed-off-by: R SUNDAR <prosunofficial@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>