drm/amd/display: Remove unnecessary DC_FP_START/DC_FP_END

[WHY & HOW]
Remove the unnecessary DC_FP_START/DC_FP_END pair to reduce time in
preempt_disable. It also fixes "BUG: sleeping function called from
invalid context" error messages because of calling kzalloc with
GFP_KERNEL which can sleep.

Reviewed-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Ray Wu <ray.wu@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>