drm/amdgpu: fix integer overflow in amdgpu_gem_align_pitch()

drm/amdgpu: fix integer overflow in amdgpu_gem_align_pitch()

amdgpu_gem_align_pitch() is passed u32 width and cpp from dumb buffer
creation but uses signed int internally.  The round-up add and the
aligned * cpp multiplication can overflow, returning zero or a negative
pitch.  A zero pitch propagates to a zero-sized GEM object allocation
that reaches userspace via DRM_IOCTL_MODE_CREATE_DUMB.

Switch the helper to unsigned int and use check_add_overflow() /
check_mul_overflow() so wraparound returns zero.  Reject a zero pitch
or size in amdgpu_mode_dumb_create() rather than allocating a zero-
byte BO.

Fixes: 8e911ab770f7 ("drm: amdgpu: Replace drm_fb_get_bpp_depth() with drm_format_plane_cpp()")
Signed-off-by: Werner Kasselman <werner@verivus.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>