x86, EFI: Calculate the EFI framebuffer size instead of trusting the firmware

Seth Forshee reported that his system was reporting that the EFI framebuffer
stretched from 0x90010000-0xb0010000 despite the GPU's BAR only covering
0x90000000-0x9ffffff. It's safer to calculate this value from the pixel
stride and screen height (values we already depend on) rather than face
potential problems with resource allocation later on.
Signed-off-by: Matthew Garrett <mjg@redhat.com>
Tested-by: Seth Forshee <seth.forshee@canonical.com>
Signed-off-by: Matt Fleming <matt.fleming@intel.com>

arch/x86/boot/compressed/eboot.c

@@ -331,7 +331,6 @@ static efi_status_t setup_gop(struct screen_info *si, efi_guid_t *proto,
 	si->lfb_width = width;
 	si->lfb_height = height;
 	si->lfb_base = fb_base;
-	si->lfb_size = fb_size;
 	si->pages = 1;
 
 	if (pixel_format == PIXEL_RGB_RESERVED_8BIT_PER_COLOR) {
@@ -379,6 +378,8 @@ static efi_status_t setup_gop(struct screen_info *si, efi_guid_t *proto,
 		si->rsvd_pos = 0;
 	}
 
+	si->lfb_size = si->lfb_linelength * si->lfb_height;
+
 	si->capabilities |= VIDEO_CAPABILITY_SKIP_QUIRKS;
 
 free_handle: