drm/amd/display: Remove double checks for `debug.enable_mem_low_power.bits.cm`

drm/amd/display: Remove double checks for `debug.enable_mem_low_power.bits.cm`

[Why]
A variety of the 3DLUT handling functions check
`debug.enable_mem_low_power.bits.cm` both in the caller and function.
This is unnecessary overhead.

[How]
For each of them reduce to just checking just in caller or function.

Reviewed-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Roman Li <roman.li@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>