drm/amd/display: use vmalloc for struct dc_state

This is a big structure so use vmalloc as malloc can
fail when there is memory pressure.

Closes: https://gitlab.freedesktop.org/drm/amd/-/issues/3454


Reviewed-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>