drm/amd/display: Fix maybe-uninitialized warning

drm/amd/display: Fix maybe-uninitialized warning

[Why]

Compiling with GCC 9.1.0 gives the following warning (I have
warnings-as-errors enabled):

drivers/gpu/drm/amd/amdgpu/../dal-dev/dc/core/dc.c: In function 'dc_validate_seamless_boot_timing':
drivers/gpu/drm/amd/amdgpu/../dal-dev/dc/core/dc.c:1180:8: error: 'se' may be used uninitialized in this function [-Werror=maybe-uninitialized]
 1180 |   if (!se->funcs->dp_get_pixel_format(
      |        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 1181 |    se,
      |    ~~~
 1182 |    &hw_crtc_timing.pixel_encoding,
      |    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 1183 |    &hw_crtc_timing.display_color_depth))

[How]

Initialize se to NULL.

Signed-off-by: Leo Li <sunpeng.li@amd.com>
Reviewed-by: Harry Wentland <Harry.Wentland@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>