drm/amd/display: Remove unused variables from dcn10_stream_encoder

The variable regval from the function enc1_update_generic_info_packet
and the variables dynamic_range_rgb and dynamic_range_ycbcr from the
function enc1_stream_encoder_dp_set_stream_attribute are not currently
used.

This was pointed by clang with the following warnings:

drivers/gpu/drm/amd/amdgpu/../display/dc/dcn10/dcn10_stream_encoder.c:62:11:
warning: variable 'regval' set but not used [-Wunused-but-set-variable]
        uint32_t regval;
                 ^
drivers/gpu/drm/amd/amdgpu/../display/dc/dcn10/dcn10_stream_encoder.c:262:10:
warning: variable 'dynamic_range_rgb' set but not used [-Wunused-but-set-variable]
        uint8_t dynamic_range_rgb = 0; /*full range*/
                ^
drivers/gpu/drm/amd/amdgpu/../display/dc/dcn10/dcn10_stream_encoder.c:263:10:
warning: variable 'dynamic_range_ycbcr' set but not used [-Wunused-but-set-variable]
        uint8_t dynamic_range_ycbcr = 1; /*bt709*/
                ^
3 warnings generated.

Reviewed-by: André Almeida <andrealmeid@igalia.com>
Signed-off-by: Maíra Canal <mairacanal@riseup.net>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>