MFC r306557, r307240, r307241:

r306557:
Use VM_MEMATTR_WRITE_COMBINING memattr for mmap(2) on framebuffer

VM_MEMATTR_WRITE_COMBINING sets write-through cache flag for framebuffer
memory that prevents pixel data from being stuck in cache until evicition
happens

r307240:
Fix BCM238x framebuffer driver build for ARM64

VM_MEMATTR_WRITE_COMBINING can be undefined for some platforms, use it only
if it's defined

r307241:
Do not set FB_FLAG_MEMATTR if VM_MEMATTR_WRITE_COMBINING is not available

Pintyhat to: gonzo
Spotted by: jmallett