modesetting: code refactor for PRIME sync

The X will be crashed on the system with other DDX driver,
such as amdgpu.

show the log like:

randr: falling back to unsynchronized pixmap sharing
(EE)
(EE) Backtrace:
(EE) 0: /usr/lib/xorg/Xorg (xorg_backtrace+0x4e)
(EE) 1: /usr/lib/xorg/Xorg (0x55cb0151a000+0x1b5ce9)
(EE) 2: /lib/x86_64-linux-gnu/libpthread.so.0 (0x7f1587a1d000+0x11390)
(EE)
(EE) Segmentation fault at address 0x0
(EE)

The issue is that modesetting as the master, and amdgpu as the slave.
Thus, when the master attempts to access pSlavePixPriv in ms_dirty_update(),
problems result due to the fact that it's accessing AMD's 'ppriv' using the
modesetting structure definition.

Apart from fixing crash issue, the patch fix other issue in master interface
in which driver should refer to master pixmap.

Signed-off-by: Jim Qu <Jim.Qu@amd.com>
Reviewed-by: Alex Goins <agoins@nvidia.com>
(cherry picked from commit f79e5368)