drm/amd/display: Use kvzalloc for potentially large allocations

Allocating up to 32 physically contiguous pages can easily fail (and has
failed for me), and isn't necessary anyway.

Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>