x86/mm: Refactor __set_clr_pte_enc()

Refactor __set_clr_pte_enc() and add two new helper functions to
set/clear PTE C-bit from early SEV/SNP initialization code and later
during shutdown/kexec especially when all CPUs are stopped and
interrupts are disabled and set_memory_xx() interfaces can't be used.

Co-developed-by: Borislav Petkov (AMD) <bp@alien8.de>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Signed-off-by: Ashish Kalra <ashish.kalra@amd.com>
Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
Link: https://lore.kernel.org/r/5df4aa450447f28294d1c5a890e27b63ed4ded36.1722520012.git.ashish.kalra@amd.com