MFC r352028:

[rpi] Inherit framebuffer BPP value from the VideoCore firmware

Instead of using hardcoded bpp of 24, obtain current/configured value
from VideoCore. This solves certain problems with Xorg/Qt apps that
require bpp of 32 to work properly. The mode can be forced by setting
framebuffer_depth value in config.txt

PR: 235363
Submitted by: Steve Peurifoy <ssw01@mathistry.net>
Tested by: Johnathan Chen <jonc@chen.org.nz> (stabe/12 patch)