drm/amd/display: check fb of primary plane

drm/amd/display: check fb of primary plane

Sometimes the primary plane might not be initialized (yet), which
causes dm_check_crtc_cursor to divide by zero.
Apparently a weird state before a S3-suspend causes the aforementioned
divide-by-zero error when resuming from S3.  This was explained in
bug 212293 on Bugzilla.

To avoid this divide-by-zero error we check if the primary plane's fb
isn't NULL.  If it's NULL the src_w and src_h attributes will be 0,
which would cause a divide-by-zero.

This fixes Bugzilla report 212293
Bug: https://bugzilla.kernel.org/show_bug.cgi?id=212293

Fixes: 12f4849a1cfd69f3 ("drm/amd/display: check cursor scaling")
Reviewed-by: Simon Ser <contact@emersion.fr>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Sefa Eyeoglu <contact@scrumplex.net>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>