drm/amd/display: Use local variable instead of caching global

[Why]
context->bw.dce.dispclk_khz is being cached into unpatched_clock, then
restored at end of function call. This is needlessly complex

[How]
Instead, use a local patched_clock variable. Leave
context->bw.dce.dispclk_khz alone.

No functional change is intended.

Signed-off-by: David Francis <david.francis@amd.com>
Signed-off-by: Leo Li <sunpeng.li@amd.com>
Reviewed-by: Harry Wentland <Harry.Wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>