drm/amd/display: Remove unused variable

Remove the variable clamshell_closed from the function
dcn10_align_pixel_clocks.

This was pointed by clang with the following warning:

drivers/gpu/drm/amd/amdgpu/../display/dc/dcn10/dcn10_hw_sequencer.c:2063:7:
warning: variable 'clamshell_closed' set but not used
[-Wunused-but-set-variable]
    bool clamshell_closed = false;
         ^

Signed-off-by: Maíra Canal <maira.canal@usp.br>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>