DDB minimum requirement of crtc configuration (cumulative of all the
enabled planes in crtc) may exceed the allocated DDB for crtc/pipe.
This patch make changes to fail the flip/ioctl if minimum requirement
for pipe exceeds the total ddb allocated to the pipe.
Previously it succeeded but making alloc_size a negative value. Which
will make subsequent calculations for plane ddb allocation bogus & may
lead to screen corruption or system hang.

Changes from V1:
 - Improve commit message as per Ander's comment
 - Remove extra parentheses (Ander)
Signed-off-by: NMahesh Kumar <mahesh1.kumar@intel.com>
Reviewed-by: NMatt Roper <matthew.d.roper@intel.com>
Reviewed-by: NAnder Conselvan de Oliveira <conselvan2@gmail.com>
Reviewed-by: NMaarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: NMatt Roper <matthew.d.roper@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170517115831.13830-8-mahesh1.kumar@intel.com