drm/amd/display: validate plane degamma LUT size for private color prop

In the Linux kernel, the following vulnerability has been resolved: drm/amd/display: validate plane degamma LUT size for private color prop Unlike the CRTC degamma path, which is guarded by amdgpu_dm_verify_lut_sizes(), the per-plane degamma LUT size was never validated before use. __set_dm_plane_degamma() passed the user-supplied size straight into __is_lut_linear() and, for a non-linear LUT, into __set_input_tf() -> __drm_lut_to_dc_gamma(), the latter always iterating MAX_COLOR_LUT_ENTRIES entries regardless of the actual LUT size. A malformed AMD_PLANE_DEGAMMA_LUT blob (e.g. a single entry) could thus trigger a divide-by-zero in __is_lut_linear() or an out-of-bounds read in __drm_lut_to_dc_gamma(). Reject any plane degamma LUT whose size does not match MAX_COLOR_LUT_ENTRIES, mirroring the invariant the code already asserts a few lines below (and which the CRTC path enforces). The AMD_PLANE_DEGAMMA_LUT property is only exposed on builds with AMD_PRIVATE_COLOR defined.