drm/amdgpu/display: Initialize gamma correction mode variable in dcn30_get_gamcor_current()

The dcn30_get_gamcor_current() function is responsible for determining
the current gamma correction mode used by the display controller.
However, the 'mode' variable, which stores the gamma correction mode,
was not initialized before its first usage, leading to an uninitialized
symbol error.

Thus initializes the 'mode' variable with a default value of LUT_BYPASS
before the conditional statements in the function, improves code clarity
and stability, ensuring correct behavior of the
dcn30_get_gamcor_current() function in determining the gamma correction
mode.

Fixes the below:
drivers/gpu/drm/amd/amdgpu/../display/dc/dcn30/dcn30_dpp_cm.c:77 dpp30_get_gamcor_current() error: uninitialized symbol 'mode'.

Fixes: 03f54d7d ("drm/amd/display: Add DCN3 DPP")
Cc: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Cc: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Cc: Aurabindo Pillai <aurabindo.pillai@amd.com>
Cc: Tom Chung <chiahsuan.chung@amd.com>
Signed-off-by: Srinivasan Shanmugam <srinivasan.shanmugam@amd.com>
Suggested-by: Roman Li <roman.li@amd.com>
Reviewed-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>