drm/i915/dmc: explicitly sanitize num_entries from package_header

drm/i915/dmc: explicitly sanitize num_entries from package_header

num_entries comes from package_header, which is read from an external
firmware blob and thus untrusted.  In parse_dmc_fw_package() we assign
package_header->num_entries to a local variable, but the range check
still uses the struct field directly.

Switch the check to use the local copy instead.  This makes the
sanitization explicit and avoids a redundant dereference.

Reviewed-by: Mitul Golani <mitulkumar.ajitkumar.golani@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Link: https://lore.kernel.org/r/20250909083042.1292672-1-luciano.coelho@intel.com